Employee benefit plans - Information about fair value of plan assets (Detail) - JPY (¥) ¥ in Millions		Mar. 31, 2017		Mar. 31, 2016		Mar. 31, 2015
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		¥ 233,945		¥ 232,027		¥ 233,837
Equities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		24,375		21,283
Private equity investments [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		6,785		7,510
Japanese government securities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		53,270		61,803
Bank and corporate debt securities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		4,321		4,543
Investment trust funds and other [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets	[1]	60,240		60,633
Life insurance company general accounts [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		53,098		52,109
Other assets [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		31,856		24,146
Level 1 [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		80,034		85,466
Level 1 [Member] | Equities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		24,375		21,283
Level 1 [Member] | Private equity investments [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 1 [Member] | Japanese government securities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		53,270		61,803
Level 1 [Member] | Bank and corporate debt securities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		2,389		2,380
Level 1 [Member] | Investment trust funds and other [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets	[1]
Level 1 [Member] | Life insurance company general accounts [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 1 [Member] | Other assets [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 2 [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		96,702		91,352
Level 2 [Member] | Equities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 2 [Member] | Private equity investments [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 2 [Member] | Japanese government securities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 2 [Member] | Bank and corporate debt securities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		1,932		2,163
Level 2 [Member] | Investment trust funds and other [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets	[1]	9,816		12,934
Level 2 [Member] | Life insurance company general accounts [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		53,098		52,109
Level 2 [Member] | Other assets [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		31,856		24,146
Level 3 [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		57,209		55,209		55,338
Level 3 [Member] | Equities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 3 [Member] | Private equity investments [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		6,785		7,510		6,793
Level 3 [Member] | Japanese government securities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 3 [Member] | Bank and corporate debt securities [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 3 [Member] | Investment trust funds and other [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets		50,424	[1]	47,699	[1]	¥ 48,545
Level 3 [Member] | Life insurance company general accounts [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets
Level 3 [Member] | Other assets [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair value of plan assets

[1] Includes hedge funds and real estate funds.