Summary of Rating of Debt securities (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 15,618,470	$ 14,211,745
Percentage of debt securities		100.00%
AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 2,067,500
Percentage of debt securities		13.20%
AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 7,527,600
Percentage of debt securities		48.20%
A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 3,175,900
Percentage of debt securities		20.30%
BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 2,413,800
Percentage of debt securities		15.50%
Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	$ 433,700
Percentage of debt securities	[1]	2.80%
U.S. Government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 1,360,100	1,215,000
U.S. Government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		5,900
U.S. Government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,354,200
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		2,656,800	2,307,900
Municipal bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		225,900
Municipal bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,869,600
Municipal bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		467,500
Municipal bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		86,400
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		879,500	690,700
Foreign government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		385,800
Foreign government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		405,000
Foreign government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		87,500
Foreign government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,200
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		3,545,700	3,359,000
U.S. corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		8,700
U.S. corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		137,300
U.S. corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,473,100
U.S. corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,553,800
U.S. corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	372,800
Foreign corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,323,100	1,377,400
Foreign corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		235,300
Foreign corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		103,200
Foreign corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		571,500
Foreign corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		382,300
Foreign corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	30,800
RMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	2,611,400	1,840,400
RMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,500
RMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,602,300
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]	5,600
CMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		890,300	871,700
CMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		279,400
CMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		408,400
CMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		200,000
CMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,500
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]	2,351,600	$ 2,549,600
Other asset-backed securities | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		924,000
Other asset-backed securities | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		647,600
Other asset-backed securities | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		376,300
Other asset-backed securities | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		386,600
Other asset-backed securities | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	$ 17,100

[1] Consists of $158.6 million of securities rated BB / Ba, $191.8 million of securities rated B, $26.0 million of securities rated CCC, $0.5 million of securities rated CC, $4.5 million of securities rated below CC and $52.3 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 $878.4 million and $834.2 million of collateralized loan obligations as of December 31, 2020 and 2019, respectively.