Schedule of Investment Income (Parenthetical) (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Investment expense	[1]	$ 5,168	$ 3,309	$ 3,599
Commitment to purchase alternative investment		40,000		$ 50,000	$ 40,000
Upfront Fees
Schedule of Available-for-sale Securities [Line Items]
Investment expense		$ 1,500

[1] Investment expense for the year ended December 31, 2016 includes $1.5 million in upfront fees necessary to enter into a new investment. See Note 9 for additional information on the Company's $40 million commitment related to this new investment.