Accumulated Other Comprehensive (Loss) Income - Summary of the Tax Effects Allocated to Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Unrealized (loss) gain on investments:
Reclassification adjustment for loss (gain) included in earnings	$ 104	$ (40)	$ (10,757)	$ (40)
Reclassification adjustment for loss (gain) included in earnings, tax benefit or (expense)		15		15
Reclassification adjustment for loss (gain) included in earnings net-of-tax		(25)		(25)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, before tax	403	0	769	0
Other comprehensive income (loss), before-tax amount	(172)	3,217	(6,017)	3,219
Other comprehensive income (loss), tax benefit or (expense)	(75)	(1,238)	2,319	(1,239)
Total other comprehensive income (loss), net of tax	(247)	1,979	(3,698)	1,980
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized (loss) gain on investments:
Reclassification adjustment for loss (gain) included in earnings	104		(10,757)
Reclassification adjustment for loss (gain) included in earnings, tax benefit or (expense)	(40)		4,143
Reclassification adjustment for loss (gain) included in earnings net-of-tax	64		(6,614)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, before tax	403		769
Reclassification adjustment for other-than-temporary impairment loss included in earnings, tax benefit or (expense)	(296)		(296)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, net of tax	107		473
Other comprehensive income (loss), before-tax amount	(172)		(6,017)
Other comprehensive income (loss), tax benefit or (expense)	(75)		2,319
Total other comprehensive income (loss), net of tax	(247)		(3,698)
Available-for-sale investments \| Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized (loss) gain on investments:
Unrealized (loss) gain on investments, before-tax amount	(683)	3,257	3,967	3,259
Unrealized (loss) gain on investments, tax benefit or (expense)	263	(1,253)	(1,526)	(1,254)
Unrealized (loss) gain on investments, net-of-tax amount	(420)	$ 2,004	2,441	$ 2,005
Restricted investments \| Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized (loss) gain on investments:
Unrealized (loss) gain on investments, before-tax amount	4		4
Unrealized (loss) gain on investments, tax benefit or (expense)	(2)		(2)
Unrealized (loss) gain on investments, net-of-tax amount	$ 2		$ 2

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