Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Detail) - JPY (¥) ¥ in Millions		Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		¥ 270,120	¥ 234,117
Derivative assets		42,383	10,486
Derivative liabilities		35,866	32,822
Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		16,253	4,688
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		270,120	234,117
Other investments	[1]	17,220	13,914
Derivative assets	[2]	42,383	10,486
Total assets		4,774,922	4,079,047
Future insurance policy benefits and policyholders' account in the life insurance business	[3]	532,191
Derivative liabilities	[2]	35,866	32,822
Total liabilities		568,057	32,822
Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,761,929	1,643,589
Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		69,172	67,497
Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		220,709	219,388
Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	279,777	161,495
Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	359,750	360,867
Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[6]	205,223	190,112
Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,532,386	1,172,894
Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		16,253	4,688
Marketable securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		270,120	234,117
Other investments	[1]	0
Derivative assets	[2]	0
Total assets		1,841,697	1,318,251
Derivative liabilities	[2]	0
Total liabilities		64,045
Marketable securities | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		10,011	18,719
Marketable securities | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		15,334	7,768
Marketable securities | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		14,774	11,472
Marketable securities | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	2,690	3,984
Marketable securities | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	94,156	90,801
Marketable securities | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[6]	0
Marketable securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,434,612	951,390
Marketable securities | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Securities investments and other | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		0
Other investments	[1]	17,220	13,914
Derivative assets	[2]	0
Total assets		2,890,842	2,750,310
Derivative liabilities	[2]	0
Total liabilities		468,146
Securities investments and other | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,751,918	1,624,870
Securities investments and other | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		53,838	59,729
Securities investments and other | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		205,935	207,916
Securities investments and other | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	277,087	157,511
Securities investments and other | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	265,594	270,066
Securities investments and other | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[6]	205,223	190,112
Securities investments and other | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		97,774	221,504
Securities investments and other | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		16,253	4,688
Future insurance policy benefits | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Future insurance policy benefits and policyholders' account in the life insurance business	[3]	64,045
Policyholders' account | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Future insurance policy benefits and policyholders' account in the life insurance business	[3]	468,146
Other current assets and liabilities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		0
Other investments	[1]	0
Derivative assets	[2]	40,784	9,431
Total assets		40,784	9,431
Future insurance policy benefits and policyholders' account in the life insurance business	[3]	0
Derivative liabilities	[2]	16,814	19,852
Total liabilities		16,814	19,852
Other current assets and liabilities | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other current assets and liabilities | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other current assets and liabilities | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other current assets and liabilities | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	0
Other current assets and liabilities | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	0
Other current assets and liabilities | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[6]	0
Other current assets and liabilities | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other current assets and liabilities | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		0
Other investments	[1]	0
Derivative assets	[2]	1,599	1,055
Total assets		1,599	1,055
Future insurance policy benefits and policyholders' account in the life insurance business	[3]	0
Derivative liabilities	[2]	19,052	12,970
Total liabilities		19,052	12,970
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	0
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	0
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[6]	0
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Other noncurrent assets and liabilities | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		24,330	22,105
Other investments	[1]	7,162	5,489
Derivative assets	[2]	1,310	444
Total assets		983,546	1,065,138
Future insurance policy benefits and policyholders' account in the life insurance business	[3]	0
Derivative liabilities	[2]	2,077	136
Total liabilities		2,077	136
Level 1 | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Level 1 | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Level 1 | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Level 1 | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	0
Level 1 | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	0
Level 1 | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[6]	0
Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		950,744	1,037,100
Level 1 | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		245,790	212,012
Other investments	[1]	816	1,507
Derivative assets	[2]	41,073	10,042
Total assets		3,582,393	2,819,204
Future insurance policy benefits and policyholders' account in the life insurance business	[3]	0
Derivative liabilities	[2]	33,789	32,686
Total liabilities		33,789	32,686
Level 2 | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1,761,929	1,643,589
Level 2 | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		69,172	67,497
Level 2 | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		220,679	219,388
Level 2 | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	279,777	161,495
Level 2 | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	343,980	338,163
Level 2 | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[6]	33,383	25,029
Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		581,642	135,794
Level 2 | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		4,152	4,688
Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities		0
Other investments	[1]	9,242	6,918
Derivative assets	[2]	0
Total assets		208,983	194,705
Future insurance policy benefits and policyholders' account in the life insurance business	[3]	532,191
Derivative liabilities	[2]	0
Total liabilities		532,191
Level 3 | Fair Value, Measurements, Recurring | Japanese national government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Level 3 | Fair Value, Measurements, Recurring | Japanese local government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Level 3 | Fair Value, Measurements, Recurring | Japanese corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		30
Level 3 | Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[4]	0
Level 3 | Fair Value, Measurements, Recurring | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[5]	15,770	22,704
Level 3 | Fair Value, Measurements, Recurring | Securitized products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[6]	171,840	¥ 165,083
Level 3 | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Level 3 | Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		¥ 12,101

[1] Other investments include certain hybrid financial instruments and certain private equity investments.

[2] Derivative assets and liabilities are recognized and disclosed on a gross basis.

[3] Future insurance policy benefits and policyholders’ account in the life insurance business are those for which the fair value option has been elected.

[4] 4,910 million yen and 7,771 million yen are included in foreign securities for which the fair value option has been elected and classified in level 2 for the fiscal years ended March 31, 2019 and 2020. In the consolidated balance sheets, 2,386 million yen are included as marketable securities for the fiscal years ended March 31, 2020 and 4,910 million yen and 5,385 million yen are included as securities investment and other for the fiscal years ended March 31, 2019 and 2020, respectively.

[5] 173,964 million yen and 188,426 million yen are included in foreign securities for which the fair value option has been elected and classified in level 2 for the fiscal years ended March 31, 2019 and 2020, respectively. In the consolidated balance sheets, 33,391 million yen and 34,502 million yen are included as marketable securities and 140,573 million yen and 153,924 million yen are included as securities investment and other for the fiscal years ended March 31, 2019 and 2020, respectively.

[6] 185,195 million yen and 193,430 million yen are included in foreign securities for which the fair value option has been elected and classified in level 2 and level 3 for the fiscal years ended March 31, 2019 and 2020, respectively, and are included in the consolidated balance sheets as securities investments and other.