Components of Net Realized Investment Gains (Losses) (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total net realized investment gains (losses)		$ 2,426	$ (963)	$ (662)	$ 775	$ 30,778	$ 1,928	$ (3,492)	$ (7,493)	$ 1,576	$ 21,721	$ (3,374)
Not Designated as Hedging Instrument | Interest Rate Swap
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains										3,555	3,733
Gross realized losses										(3,630)	(4,843)	(6,988)
Total net realized investment gains (losses)	[1]									(75)	(1,110)	(6,988)
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains										4,066	2,947	3,565
Gross realized losses										(3,387)	(691)	(2,180)
Total net realized investment gains (losses)										679	2,256	1,385
Common Shares
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains										4,178	28,785	10,379
Gross realized losses										(3,206)	(8,210)	(8,246)
Total net realized investment gains (losses)										$ 972	$ 20,575	2,133
Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains												96
Total net realized investment gains (losses)												$ 96

[1] Includes $3.6 million, $4.8 million, and $5.4 million of periodic net interest settlements related to the derivatives for the years ended December 31, 2017, 2016, and 2015, respectively.