Investments - Summary of Rating of Debt securities (Detail) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2022	Dec. 31, 2021
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 14,651,254	$ 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		$ 2,538,500
Percentage of debt securities, before allowance for credit losses		17.30%
AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 6,465,300
Percentage of debt securities, before allowance for credit losses		44.10%
A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 3,138,600
Percentage of debt securities, before allowance for credit losses		21.50%
BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 2,083,400
Percentage of debt securities, before allowance for credit losses		14.20%
Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	$ 425,500
Percentage of debt securities, before allowance for credit losses		2.90%
U.S. Government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 1,538,500	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		1,537,200
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		2,626,800	2,535,900
Municipal bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		293,200
Municipal bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,759,100
Municipal bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		490,300
Municipal bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		76,300
Municipal bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	7,900
Foreign government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		744,200	854,900
Foreign government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		325,500
Foreign government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		345,700
Foreign government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		72,000
Foreign government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,000
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		3,157,200	3,477,300
U.S. corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		12,900
U.S. corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		142,400
U.S. corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,437,500
U.S. corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,249,100
U.S. corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	315,300
Foreign corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,085,100	1,226,900
Foreign corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		142,600
Foreign corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		59,500
Foreign corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		514,000
Foreign corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		336,600
Foreign corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	32,400
RMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[2]	1,668,700	2,009,000
RMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		5,300
RMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,657,500
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		700
RMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	4,800
CMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		925,700	905,900
CMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		373,800
CMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		352,300
CMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		169,700
CMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		29,800
CMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	100
Other asset-backed securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[3],[4]	2,905,100	$ 3,001,000
Other asset-backed securities | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,383,900
Other asset-backed securities | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		611,600
Other asset-backed securities | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		454,700
Other asset-backed securities | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		389,900
Other asset-backed securities | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	$ 65,000

[1] Consists of $124.4 million of securities rated BB / Ba, $173.7 million of securities rated B, $21.2 million of securities rated CCC, $0.5 million of securities rated CC, $2.9 million of securities rated below CC and $102.8 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 $1,229.6 million and $1,320.4 million of collateralized loan obligations as of June 30, 2022 and December 31, 2021, respectively.

[4] Includes $1,229.6 million and $1,320.4 million of collateralized loan obligations as of June 30, 2022 and December 31, 2021, respectively.