Summary of Rating of Debt securities (Detail) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2020	Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 15,579,894	$ 14,211,745
Percentage of debt securities		100.00%
AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 1,988,300
Percentage of debt securities		12.80%
BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 2,344,900
Percentage of debt securities		15.00%
AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 7,255,000
Percentage of debt securities		46.60%
A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 3,479,000
Percentage of debt securities		22.30%
Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	$ 512,700
Percentage of debt securities	[1]	3.30%
U.S. government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 1,238,400	1,215,000
U.S. government obligations | AAA / Aaa
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 | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,238,400
U.S. government obligations | A
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		2,561,100	2,307,900
Municipal bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		222,400
Municipal bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		95,700
Municipal bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,772,200
Municipal bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		463,800
Municipal bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	7,000
Foreign government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		800,100	690,700
Foreign government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		343,000
Foreign government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,100
Foreign government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		359,000
Foreign government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		97,000
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		3,958,000	3,359,000
U.S. corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		9,500
U.S. corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,580,200
U.S. corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		284,200
U.S. corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,715,900
U.S. corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	368,200
Foreign corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,305,500	1,377,400
Foreign corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		238,500
Foreign corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		324,800
Foreign corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		94,500
Foreign corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		612,100
Foreign corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	35,600
RMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	2,511,900	1,840,400
RMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,500
RMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,000
RMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,503,900
RMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		0
RMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	4,500
CMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		883,200	871,700
CMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		280,600
CMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		300
CMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		420,800
CMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		181,500
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]	2,321,700	$ 2,549,600
Other asset-backed securities | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		891,800
Other asset-backed securities | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		341,800
Other asset-backed securities | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		582,000
Other asset-backed securities | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		408,700
Other asset-backed securities | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[1]	$ 97,400

[1] Consists of $138.6 million of securities rated BB / Ba, $181.7 million of securities rated B, $22.9 million of securities rated CCC, $0.4 million of securities rated CC, $3.5 million of securities rated below CC and $165.6 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 $760.5 million and $834.2 million of collateralized loan obligations as of September 30, 2020 and December 31, 2019, respectively.