Summary of Aggregate Fair Value and Gross Unrealized Loss by Length of Time Securities had Continuously been in Gross Unrealized Loss Position as Well as Number of Securities (Detail) - Fixed Maturities - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		$ 434,545	$ 1,216,029
Less than 12 months Gross Unrealized Losses		(4,528)	(27,511)
Greater than 12 months Fair Value		1,081,356	917,659
Greater than 12 months Gross Unrealized Losses		(21,941)	(33,038)
Fair Value		1,515,901	2,133,688
Gross Unrealized Losses		(26,469)	(60,549)
U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		32,603	101,395
Less than 12 months Gross Unrealized Losses		(377)	(2,486)
Greater than 12 months Fair Value		121,784	112,195
Greater than 12 months Gross Unrealized Losses		(2,673)	(1,471)
Fair Value		154,387	213,590
Gross Unrealized Losses		(3,050)	(3,957)
States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		6,964	123,479
Less than 12 months Gross Unrealized Losses		(4)	(1,274)
Greater than 12 months Fair Value		76,493	105,405
Greater than 12 months Gross Unrealized Losses		(582)	(2,174)
Fair Value		83,457	228,884
Gross Unrealized Losses		(586)	(3,448)
Agency Residential Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		2,924	34,819
Less than 12 months Gross Unrealized Losses		(19)	(814)
Greater than 12 months Fair Value		291,576	270,997
Greater than 12 months Gross Unrealized Losses		(8,752)	(12,304)
Fair Value		294,500	305,816
Gross Unrealized Losses		(8,771)	(13,118)
Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		35,368	70,965
Less than 12 months Gross Unrealized Losses		(36)	(249)
Greater than 12 months Fair Value		14,258	10,110
Greater than 12 months Gross Unrealized Losses		(259)	(281)
Fair Value		49,626	81,075
Gross Unrealized Losses		(295)	(530)
Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		266,463	345,437
Less than 12 months Gross Unrealized Losses		(3,490)	(6,950)
Greater than 12 months Fair Value		96,222	77,362
Greater than 12 months Gross Unrealized Losses		(819)	(1,090)
Fair Value		362,685	422,799
Gross Unrealized Losses		(4,309)	(8,040)
Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		55,185	58,877
Less than 12 months Gross Unrealized Losses		(141)	(282)
Greater than 12 months Fair Value		42,717	41,477
Greater than 12 months Gross Unrealized Losses		(1,256)	(2,534)
Fair Value		97,902	100,354
Gross Unrealized Losses		(1,397)	(2,816)
Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		359,940	510,098
Less than 12 months Gross Unrealized Losses		(3,686)	(8,295)
Greater than 12 months Fair Value		444,773	399,946
Greater than 12 months Gross Unrealized Losses		(11,086)	(16,209)
Fair Value		804,713	910,044
Gross Unrealized Losses		(14,772)	(24,504)
Corporate Exposures
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	[1]	35,038	481,057
Less than 12 months Gross Unrealized Losses	[1]	(461)	(15,456)
Greater than 12 months Fair Value	[1]	438,306	300,113
Greater than 12 months Gross Unrealized Losses	[1]	(7,600)	(13,184)
Fair Value	[1]	473,344	781,170
Gross Unrealized Losses	[1]	$ (8,061)	$ (28,640)

[1] Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.