Summary of Rating of Debt securities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2022	Dec. 31, 2021
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 15,558,624	$ 16,061,560
Percentage of debt securities, before allowance for credit losses		100.00%
AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 2,726,800
Percentage of debt securities, before allowance for credit losses		17.50%
AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 6,865,500
Percentage of debt securities, before allowance for credit losses		44.10%
A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 3,229,900
Percentage of debt securities, before allowance for credit losses		20.80%
BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 2,291,800
Percentage of debt securities, before allowance for credit losses		14.70%
Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	$ 444,600
Percentage of debt securities, before allowance for credit losses	[1]	2.90%
U.S. Government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 1,593,000	2,050,700
U.S. Government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,300
U.S. Government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,591,700
U.S. Government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		0
U.S. Government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		0
U.S. Government obligations | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	0
Municipal bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		2,641,900	2,535,900
Municipal bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		279,300
Municipal bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,813,900
Municipal bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		462,800
Municipal bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		78,100
Municipal bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	7,800
Foreign government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		818,000	854,900
Foreign government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		361,000
Foreign government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		371,800
Foreign government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		84,100
Foreign government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,100
Foreign government obligations | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	0
U.S. corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		3,350,800	3,477,300
U.S. corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		14,500
U.S. corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		158,300
U.S. corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,429,000
U.S. corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,416,900
U.S. corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	332,100
Foreign corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,186,400	1,226,900
Foreign corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		156,600
Foreign corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		65,000
Foreign corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		573,100
Foreign corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		355,700
Foreign corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	36,000
RMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[2]	1,831,900	2,009,000
RMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		6,200
RMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,819,300
RMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		400
RMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		800
RMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	5,200
CMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,048,700	905,900
CMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		447,600
CMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		368,000
CMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		200,600
CMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		32,500
CMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	0
Other asset-backed securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[3],[4]	3,087,900	$ 3,001,000
Other asset-backed securities | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,460,300
Other asset-backed securities | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		677,500
Other asset-backed securities | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		479,900
Other asset-backed securities | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		406,700
Other asset-backed securities | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	$ 63,500

[1] Consists of $130.6 million of securities rated BB / Ba, $188.9 million of securities rated B, $31.3 million of securities rated CCC, $0.5 million of securities rated CC, $2.9 million of securities rated below CC and $90.4 million of not rated securities.

[2] Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.

[3] Includes $1,281.5 million and $1,320.4 million of collateralized loan obligations as of March 31, 2022 and December 31, 2021, respectively.

[4] Includes $1,281.5 million and $1,320.4 million of collateralized loan obligations as of March 31, 2022 and December 31, 2021, respectively.