Summary of Rating of Debt securities (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 16,061,560	$ 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, fair value		$ 2,406,000
Percentage of debt securities, before allowance for credit losses		15.00%
AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 7,544,400
Percentage of debt securities, before allowance for credit losses		47.00%
A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 3,259,900
Percentage of debt securities, before allowance for credit losses		20.30%
BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 2,402,400
Percentage of debt securities, before allowance for credit losses		14.90%
Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	$ 448,900
Percentage of debt securities, before allowance for credit losses	[1]	2.80%
U.S. Government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		$ 2,050,700	1,360,100
U.S. Government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,400
U.S. Government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		2,049,300
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		2,535,900	2,656,800
Municipal bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		219,000
Municipal bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,756,300
Municipal bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		469,200
Municipal bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		83,300
Municipal bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	8,100
Foreign government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		854,900	879,500
Foreign government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		376,500
Foreign government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		389,300
Foreign government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		87,800
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]	200
U.S. corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		3,477,300	3,545,700
U.S. corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		13,900
U.S. corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		165,600
U.S. corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,442,900
U.S. corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,485,100
U.S. corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	369,800
Foreign corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,226,900	1,323,100
Foreign corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		184,700
Foreign corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		69,100
Foreign corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		564,100
Foreign corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		372,800
Foreign corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	36,200
RMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[2]	2,009,000	2,611,400
RMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		4,200
RMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,998,400
RMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		0
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]	5,600
CMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		905,900	890,300
CMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		324,200
CMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		355,400
CMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		218,600
CMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		7,700
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]	3,001,000	$ 2,351,600
Other asset-backed securities | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		1,282,100
Other asset-backed securities | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		761,000
Other asset-backed securities | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		477,300
Other asset-backed securities | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value		451,600
Other asset-backed securities | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities, fair value	[1]	$ 29,000

[1] Consists of $140.8 million of securities rated BB / Ba, $203.5 million of securities rated B, $32.0 million of securities rated CCC, $0.5 million of securities rated CC, $3.1 million of securities rated below CC and $69.0 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 $1,320.4 million and $878.4 million of collateralized loan obligations as of December 31, 2021 and 2020, respectively.