Accumulated Other Comprehensive Loss - Summary of the Tax Effects Allocated to Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Unrealized loss on investments - debt securities and restricted investments:
Reclassification adjustment for loss (gain) included in earnings, before-tax-amount		$ 1,078	$ (3,122)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, before tax		63	366
Reclassification into retained earnings for the adoption of ASU 2016-01, before-tax-amount	[1]	932	0
Reclassification into retained earnings for the adoption of ASU 2018-02, tax benefit or (expense)		300
Other comprehensive income (loss), before-tax amount		1,261	1,153
Other comprehensive income (loss), tax benefit or (expense)		(632)	(441)
Total other comprehensive income (loss), net of tax		629	712
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized loss on investments - debt securities and restricted investments:
Reclassification adjustment for loss (gain) included in earnings, before-tax-amount		1,078	(3,122)
Reclassification adjustment for loss (gain) included in earnings, tax benefit or (expense)		(273)	1,203
Reclassification adjustment for loss (gain) included in earnings net-of-tax		805	(1,919)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, before tax		63	366
Reclassification adjustment for other-than-temporary impairment loss included in earnings, tax benefit or (expense)		(16)	(140)
Reclassification adjustment for other-than-temporary impairment loss included in earnings, net of tax		47	226
Reclassification into retained earnings for the adoption of ASU 2016-01, before-tax-amount		932
Reclassification into retained earnings for the adoption of ASU 2016-01, tax benefit or (expense)		(236)
Reclassification into retained earnings for the adoption of ASU 2016-01, net-of-tax		696
Reclassification into retained earnings for the adoption of ASU 2018-02, before-tax-amount		0
Reclassification into retained earnings for the adoption of ASU 2018-02, tax benefit or (expense)		(313)
Reclassification into retained earnings for the adoption of ASU 2018-02 , net-of-tax		(313)
Other comprehensive income (loss), before-tax amount		1,261	1,153
Other comprehensive income (loss), tax benefit or (expense)		(632)	(441)
Total other comprehensive income (loss), net of tax		629	712
Available-for-sale investments | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized loss on investments - debt securities and restricted investments:
Unrealized (loss) gain on investments, before-tax amount		(803)	3,905
Unrealized (loss) gain on investments, tax benefit or (expense)		204	(1,503)
Unrealized (loss) gain on investments, net-of-tax amount		(599)	2,402
Restricted investments | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Unrealized loss on investments - debt securities and restricted investments:
Unrealized (loss) gain on investments, before-tax amount		(9)	4
Unrealized (loss) gain on investments, tax benefit or (expense)		2	(1)
Unrealized (loss) gain on investments, net-of-tax amount		$ (7)	$ 3

[1] (1) The reclassification into retained earnings relates to the adoption of Accounting Standards Update (“ASU”) 2016-01 Financial Instruments - Overall, as amended (“ASU 2016-01”). The new guidance is effective January 1, 2018, and requires equity investments to be measured at fair value with changes in fair value recognized in results of operations rather than the condensed consolidated statements of comprehensive income. See Note 2. Summary of Significant Accounting Policies.