Investment in Securities - Investments in Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Bonds held-to-maturity securities, amortized cost	$ 8,576,192		$ 8,631,261
Bonds held-to-maturity securities, gross unrealized gains	207,582		345,463
Bonds held-to-maturity securities, gross unrealized (losses)	(260,733)		(8,034)
Bonds held-to-maturity securities, allowance for credit losses	(21,385)	$ 0	(21,664)
Fair value of bonds held-to-maturity	8,501,656		8,968,690
Bonds available-for-sale securities, amortized cost	6,447,577		6,435,670
Bonds available-for-sale securities, gross unrealized gains	182,107		302,426
Bonds available-for-sale securities, gross unrealized (losses)	(136,445)		(13,011)
Bonds available-for-sale securities, allowance for credit losses	(12,865)
Fixed maturity securities, bonds available-for-sale	6,480,374		6,725,085
Bonds investments in securities, cost or amortized cost	15,023,769		15,066,931
Bonds investments in securities, gross unrealized gains	389,689		647,889
Bonds investments in securities, gross unrealized (losses)	(397,178)		(21,045)
Bonds investments in securities, allowance for credit losses	(34,250)
Bonds investments in securities, fair value	14,982,030		15,693,775
U.S. states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Bonds held-to-maturity securities, amortized cost	155,151		165,109
Bonds held-to-maturity securities, gross unrealized gains	4,778		5,005
Bonds held-to-maturity securities, gross unrealized (losses)	0		0
Bonds held-to-maturity securities, allowance for credit losses	0
Fair value of bonds held-to-maturity	159,929		170,114
Bonds available-for-sale securities, amortized cost	1,028,590		1,030,309
Bonds available-for-sale securities, gross unrealized gains	50,335		47,865
Bonds available-for-sale securities, gross unrealized (losses)	(8)		(9)
Bonds available-for-sale securities, allowance for credit losses	0
Fixed maturity securities, bonds available-for-sale	1,078,917		1,078,165
U.S. treasury and government
Schedule of Held-to-maturity Securities [Line Items]
Bonds available-for-sale securities, amortized cost	29,500		29,505
Bonds available-for-sale securities, gross unrealized gains	804		441
Bonds available-for-sale securities, gross unrealized (losses)	0		(5)
Bonds available-for-sale securities, allowance for credit losses	0
Fixed maturity securities, bonds available-for-sale	30,304		29,941
Foreign governments
Schedule of Held-to-maturity Securities [Line Items]
Bonds held-to-maturity securities, amortized cost	3,894		3,907
Bonds held-to-maturity securities, gross unrealized gains	525		442
Bonds held-to-maturity securities, gross unrealized (losses)	(5)		0
Bonds held-to-maturity securities, allowance for credit losses	5		4
Fair value of bonds held-to-maturity	4,419		4,349
Bonds available-for-sale securities, amortized cost	5,000		5,000
Bonds available-for-sale securities, gross unrealized gains	1,432		1,287
Bonds available-for-sale securities, gross unrealized (losses)	0		0
Bonds available-for-sale securities, allowance for credit losses	0
Fixed maturity securities, bonds available-for-sale	6,432		6,287
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Bonds held-to-maturity securities, amortized cost	8,029,559		8,099,098
Bonds held-to-maturity securities, gross unrealized gains	193,701		332,410
Bonds held-to-maturity securities, gross unrealized (losses)	(248,247)		(6,539)
Bonds held-to-maturity securities, allowance for credit losses	(18,401)		(18,563)
Fair value of bonds held-to-maturity	7,956,612		8,424,969
Bonds available-for-sale securities, amortized cost	5,338,626		5,338,007
Bonds available-for-sale securities, gross unrealized gains	128,845		251,408
Bonds available-for-sale securities, gross unrealized (losses)	(135,711)		(12,795)
Bonds available-for-sale securities, allowance for credit losses	(12,499)
Fixed maturity securities, bonds available-for-sale	5,319,261		5,576,620
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Bonds held-to-maturity securities, amortized cost	179,087		237,516
Bonds held-to-maturity securities, gross unrealized gains	8,578		6,460
Bonds held-to-maturity securities, gross unrealized (losses)	(743)		(1,148)
Bonds held-to-maturity securities, allowance for credit losses	(3)		(137)
Fair value of bonds held-to-maturity	186,919		242,828
Bonds available-for-sale securities, amortized cost	29,599		23,405
Bonds available-for-sale securities, gross unrealized gains	229		739
Bonds available-for-sale securities, gross unrealized (losses)	(7)		(201)
Bonds available-for-sale securities, allowance for credit losses	(236)
Fixed maturity securities, bonds available-for-sale	29,585		23,943
Collateralized debt securities
Schedule of Held-to-maturity Securities [Line Items]
Bonds held-to-maturity securities, amortized cost	208,501		125,631
Bonds held-to-maturity securities, gross unrealized gains	0		1,146
Bonds held-to-maturity securities, gross unrealized (losses)	(11,738)		(347)
Bonds held-to-maturity securities, allowance for credit losses	(2,986)		(2,968)
Fair value of bonds held-to-maturity	193,777		126,430
Bonds available-for-sale securities, amortized cost	16,262		9,444
Bonds available-for-sale securities, gross unrealized gains	462		686
Bonds available-for-sale securities, gross unrealized (losses)	(719)		(1)
Bonds available-for-sale securities, allowance for credit losses	(130)
Fixed maturity securities, bonds available-for-sale	$ 15,875		$ 10,129