Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 321,692	$ 361,468
Gross Unrealized Gains (Losses)	115	3,896
Recorded Basis	321,807	365,364
Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	2,135	35,526
Recorded Basis	2,135	35,526
Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	2,135	35,526
Recorded Basis	2,135	35,526
Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	84,140	82,155
Gross Unrealized Gains (Losses)	(4,767)	435
Recorded Basis	79,373	82,590
Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	36,937	53,493
Gross Unrealized Gains (Losses)	(783)	156
Recorded Basis	36,154	53,649
Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	36,989	14,017
Gross Unrealized Gains (Losses)	(4,434)	(13)
Recorded Basis	32,555	14,004
Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	7,405	7,408
Gross Unrealized Gains (Losses)	424	282
Recorded Basis	7,829	7,690
Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	2,448	4,500
Gross Unrealized Gains (Losses)	17	9
Recorded Basis	2,465	4,509
Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	361	2,737
Gross Unrealized Gains (Losses)	9	1
Recorded Basis	370	2,738
Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	235,417	243,787
Gross Unrealized Gains (Losses)	4,882	3,461
Recorded Basis	240,299	247,248
Restricted Cash and Investments [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	34,418	29,046
Gross Unrealized Gains (Losses)	(23)
Recorded Basis	34,395	29,046
Restricted Cash and Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	99,452	98,481
Gross Unrealized Gains (Losses)	1,629	1,931
Recorded Basis	101,081	100,412
Restricted Cash and Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	291	303
Gross Unrealized Gains (Losses)	(2)
Recorded Basis	289	303
Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	25,739	27,885
Gross Unrealized Gains (Losses)	1,166	642
Recorded Basis	26,905	28,527
Restricted Cash and Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	7,271	16,906
Gross Unrealized Gains (Losses)	63	21
Recorded Basis	7,334	16,927
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	59,961	62,930
Gross Unrealized Gains (Losses)	1,978	837
Recorded Basis	61,939	63,767
Restricted Cash and Investments [Member] | Supranational Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	4,771	4,770
Gross Unrealized Gains (Losses)	71	30
Recorded Basis	4,842	4,800
Restricted Cash and Investments [Member] | Mutual Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	3,514	3,466
Recorded Basis	$ 3,514	$ 3,466