Components of Net Realized Investment Gains (Losses) (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total net realized investment gains (losses)		$ (316)	$ 775
Not Designated as Hedging Instrument | Interest Rate Swap
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains		4,801	1,236
Gross realized losses		(650)	(1,063)
Total net realized investment gains (losses)	[1]	4,151	173
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains		24	189
Gross realized losses		(117)	(83)
Total net realized investment gains (losses)		(93)	106
Common Shares
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains		3,453	575
Gross realized losses		(7,827)	(79)
Total net realized investment gains (losses)		$ (4,374)	$ 496

[1] (1) Includes $0.7 million and $1.1 million of periodic net interest settlements related to the derivatives for the quarters ended March 31, 2018 and 2017, respectively.