Accumulated Other Comprehensive (Loss) Income - Summary of Reclassifications out of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrealized gain (loss) on investments and restricted investments:
Reclassification adjustment for other-than-temporary impairment loss included in earnings before-tax amount	$ 2,288	$ 0	$ 0
Other comprehensive loss before-tax amount	(6,456)	5,196
Other comprehensive loss tax benefit or (expense)	2,488	(2,003)
Total other comprehensive (loss) income, net of tax	(3,968)	3,193	$ 639
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Unrealized gain (loss) on investments and restricted investments:
Reclassification adjustment for net gain included in earnings before-tax amount	(10,750)	(795)
Reclassification adjustment for net gain included in earnings tax (expense) or benefit	4,139	303
Reclassification adjustment for net gain included in earnings net-of-tax amount	(6,611)	(492)
Reclassification adjustment for other-than-temporary impairment loss included in earnings before-tax amount	2,288
Reclassification adjustment for other-than-temporary impairment loss included in earnings tax (expense) or benefit	(880)
Reclassification adjustment for other-than-temporary impairment loss included in earnings net-of-tax amount	1,408
Other comprehensive loss before-tax amount	(6,456)	5,196
Other comprehensive loss tax benefit or (expense)	2,488	(2,003)
Total other comprehensive (loss) income, net of tax	(3,968)	3,193
Available-for-sale Securities | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Unrealized gain (loss) on investments and restricted investments:
Unrealized gain (loss) on investments before-tax amount	2,015	5,997
Unrealized gain (loss) on investments tax (expense) or benefit	(774)	(2,308)
Unrealized gain (loss) on investments net-of-tax amount	1,241	3,689
Restricted investments | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Unrealized gain (loss) on investments and restricted investments:
Unrealized gain (loss) on investments before-tax amount	(9)	(6)
Unrealized gain (loss) on investments tax (expense) or benefit	3	2
Unrealized gain (loss) on investments net-of-tax amount	$ (6)	$ (4)