Accumulated Other Comprehensive Income (Loss) - Summary of the Tax Effects Allocated to Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Unrealized gain (loss) on investments:
Reclassification adjustment for other-than-temporary impairment loss included in earnings, before tax	$ 0	$ 0	$ 366	$ 0
Other comprehensive loss, before-tax amount	(6,998)		(5,845)
Other comprehensive loss, tax (expense) or benefit	2,835		2,394
Total other comprehensive income (loss), net of tax	(4,163)	55	(3,451)	1
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized gain (loss) on investments:
Reclassification adjustment for gain included in earnings, before tax	(7,739)	90	(10,861)
Reclassification adjustment for gain included in earnings, tax (expense) or benefit	3,120	(35)	4,183
Reclassification adjustment for gain included in earnings net-of-tax	(4,619)	$ 55	(6,678)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, before tax			366
Reclassification adjustment for other-than-temporary impairment loss included in earnings, tax (expense) or benefit			0
Reclassification adjustment for other-than-temporary impairment loss included in earnings, net of tax			366
Other comprehensive loss, before-tax amount	(6,998)		(5,845)
Other comprehensive loss, tax (expense) or benefit	2,835		2,394
Total other comprehensive income (loss), net of tax	(4,163)		(3,451)
Available-for-sale investments | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized gain (loss) on investments:
Unrealized gain (loss) on investments, before-tax amount	745		4,650	2
Unrealized gain (loss) on investments, tax (expense) or benefit	(287)		(1,789)	(1)
Unrealized gain (loss) on investments, net-of-tax amount	458		$ 2,861	$ 1
Restricted investments | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized gain (loss) on investments:
Unrealized gain (loss) on investments, before-tax amount	(4)
Unrealized gain (loss) on investments, tax (expense) or benefit	2
Unrealized gain (loss) on investments, net-of-tax amount	$ (2)