Accumulated Other Comprehensive Income (Loss) - Summary of Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unrealized gains and (losses) on investments and restricted investments:
Other comprehensive income before-tax amount	$ 5,196	$ 339
Other comprehensive income tax benefit or (expense)	(2,003)	300
Total other comprehensive income, net of tax	3,193	639	$ 6,192
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Unrealized gains and (losses) on investments and restricted investments:
Less: reclassification adjustment for gains included in earnings before-tax amount	(795)	(5,311)
Less: reclassification adjustment for gains included in earnings tax benefit or (expense)	303	2,522
Less: reclassification adjustment for gains included in earnings net-of-tax amount	(492)	(2,789)
Other comprehensive income before-tax amount	5,196	339
Other comprehensive income tax benefit or (expense)	(2,003)	300
Total other comprehensive income, net of tax	3,193	639
Available-for-sale Securities | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Unrealized gains and (losses) on investments and restricted investments:
Unrealized gains on available-for-sale investments before-tax amount	5,997	5,650
Unrealized gains on available-for-sale investments tax benefit or (expense)	(2,308)	(2,222)
Unrealized gains on available-for-sale investments net-of-tax amount	3,689	$ 3,428
Restricted investments | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Unrealized gains and (losses) on investments and restricted investments:
Unrealized gains on available-for-sale investments before-tax amount	(6)
Unrealized gains on available-for-sale investments tax benefit or (expense)	2
Unrealized gains on available-for-sale investments net-of-tax amount	$ (4)