Components of Net Realized Investment Gains (Losses) (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total net realized investment gains (losses)		$ 30,778	$ 1,928	$ (3,492)	$ (7,493)	$ 3,842	$ (10,778)	$ 6,532	$ (2,970)	$ 21,721	$ (3,374)	$ 35,860
Not Designated as Hedging Instrument | Interest Rate Swap
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains										3,733
Gross realized losses										(4,843)	(6,988)	(20,836)
Total net realized investment gains (losses)	[1]									(1,110)	(6,988)	(20,836)
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains										2,947	3,565	2,843
Gross realized losses										(691)	(2,180)	(703)
Total net realized investment gains (losses)										2,256	1,385	2,140
Common Shares
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains										28,785	10,379	55,907
Gross realized losses										(8,210)	(8,246)	(1,351)
Total net realized investment gains (losses)										$ 20,575	2,133	$ 54,556
Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains											96
Total net realized investment gains (losses)											$ 96

[1] Includes $4.8 million, $5.4 million, and $5.5 million of periodic net interest settlements related to the derivatives for the years ended December 31, 2016, 2015, and 2014, respectively.