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Investments - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities									$ (3,035)	$ 18,242	$ 27,654
Trading Securities, Realized Gain (Loss)									615	4,854	7,474
Fixed income securities										337,454	288,554
Investment expense									(6,599)	(8,098)	(9,347)
Securities available-for-sale and short-term									270,525	337,774	281,049
Total net investment income	$ 37,483	$ 58,332	$ 66,662	$ 110,240	$ 107,057	$ 87,177	$ 85,160	$ 81,559	272,717	360,953	313,367
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other Investments Income									(3,650)	13,388	20,180
Available-for-sale Securities [Member]
Investment [Line Items]
Fixed income securities									265,380
Short-term Investments [Member]
Investment [Line Items]
Fixed income securities									11,014	7,898	1,505
Financing Receivable [Member]
Investment [Line Items]
Fixed income securities									730	520	337
Other Investments [Member]
Investment [Line Items]
Fixed income securities									$ 2,192	$ 23,179	$ 32,318

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-SubTopic 10
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