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Fair Value Measurements - Summary of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis $ 289,147 $ 301,118
Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 38,056 21,312
Available-for-sale Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 251,091 279,806
Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 78 4
Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 3,061 2,946
Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 3,075 3,132
Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 220,302 251,892
Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 3,207 2,972
Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 3,432 3,612
Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Certificate of Deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 11,774 10,495
Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | U.S. Treasuries [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 6,162 4,753
Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 11,106 9,976
Level 1 [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 1 [Member] | Available-for-sale Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 11,106 9,976
Level 1 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 1 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 2,253 2,291
Level 1 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 163 241
Level 1 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 17 17
Level 1 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 2,245 2,285
Level 1 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 266 389
Level 1 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Certificate of Deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 1 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | U.S. Treasuries [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 6,162 4,753
Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 278,041 291,142
Level 2 [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 38,056 21,312
Level 2 [Member] | Available-for-sale Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 239,985 269,830
Level 2 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 78 4
Level 2 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 808 655
Level 2 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 2,912 2,891
Level 2 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 220,285 251,875
Level 2 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 962 687
Level 2 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 3,166 3,223
Level 2 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Certificate of Deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 11,774 10,495
Level 2 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | U.S. Treasuries [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member] | Available For Sale Unrestricted [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | Certificate of Deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis 0 0
Level 3 [Member] | Available-for-sale Securities [Member] | Available For Sale Restricted [Member] | U.S. Treasuries [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Recorded Basis $ 0 $ 0