Summary of Rating of Debt securities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Dec. 31, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 14,945,479	$ 15,618,470
Percentage of debt securities, before allowance for credit losses		100.00%
AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 2,016,700
Percentage of debt securities, before allowance for credit losses		13.50%
AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 6,999,600
Percentage of debt securities, before allowance for credit losses		46.80%
A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 3,048,700
Percentage of debt securities, before allowance for credit losses		20.40%
BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 2,414,100
Percentage of debt securities, before allowance for credit losses		16.20%
Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	$ 466,400
Percentage of debt securities, before allowance for credit losses	[1]	3.10%
U.S. Government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 1,525,600	1,360,100
U.S. Government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		5,700
U.S. Government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,519,900
U.S. Government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		0
U.S. Government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		0
U.S. Government obligations | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	0
Municipal bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,579,000	2,656,800
Municipal bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		213,600
Municipal bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,820,100
Municipal bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		451,900
Municipal bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		86,000
Municipal bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	7,400
Foreign government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		874,500	879,500
Foreign government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		385,100
Foreign government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		404,300
Foreign government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		84,000
Foreign government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,100
Foreign government obligations | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	0
U.S. corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		3,391,500	3,545,700
U.S. corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		8,100
U.S. corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		140,000
U.S. corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,337,000
U.S. corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,534,200
U.S. corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	372,200
Foreign corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,308,800	1,323,100
Foreign corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		212,700
Foreign corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		79,000
Foreign corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		587,800
Foreign corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		392,100
Foreign corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	37,200
RMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	1,977,100	2,611,400
RMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,700
RMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,968,000
RMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		0
RMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,000
RMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	5,400
CMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		876,700	890,300
CMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		274,800
CMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		401,500
CMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		198,000
CMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,400
CMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	0
Other asset-backed securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[3]	2,412,300	$ 2,351,600
Other asset-backed securities | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		914,000
Other asset-backed securities | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		666,800
Other asset-backed securities | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		390,000
Other asset-backed securities | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		397,300
Other asset-backed securities | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	$ 44,200

[1] Consists of $142.5 million of securities rated BB / Ba, $191.6 million of securities rated B, $25.9 million of securities rated CCC, $0.6 million of securities rated CC, $2.8 million of securities rated below CC and $103.0 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 $959.7 million and $878.4 million of collateralized loan obligations as of March 31, 2021 and December 31, 2020, respectively.