Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Total assets measured at fair value	$ 101,812	$ 36,086
Liabilities:
Total liabilities measured at fair value	249,236	247,459
Money Market Funds [Member]
Assets:
Cash Equivalents, at Carrying Value	101,812	36,086
Commitments [Member]
Liabilities:
Total liabilities measured at fair value	23,284	24,301
Accrued Liabilities [Member]
Liabilities:
Total liabilities measured at fair value	225,952	223,158
Level 1 [Member]
Assets:
Total assets measured at fair value	101,812	36,086
Liabilities:
Total liabilities measured at fair value	0	0
Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash Equivalents, at Carrying Value	101,812	36,086
Level 1 [Member] | Commitments [Member]
Liabilities:
Total liabilities measured at fair value	0	0
Level 1 [Member] | Accrued Liabilities [Member]
Liabilities:
Total liabilities measured at fair value	0	0
Level 2 [Member]
Assets:
Total assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	225,952	223,158
Level 2 [Member] | Money Market Funds [Member]
Assets:
Cash Equivalents, at Carrying Value	0	0
Level 2 [Member] | Commitments [Member]
Liabilities:
Total liabilities measured at fair value	0	0
Level 2 [Member] | Accrued Liabilities [Member]
Liabilities:
Total liabilities measured at fair value	225,952	223,158
Level 3 [Member]
Assets:
Total assets measured at fair value	0	0
Liabilities:
Total liabilities measured at fair value	23,284	24,301
Level 3 [Member] | Money Market Funds [Member]
Assets:
Cash Equivalents, at Carrying Value	0	0
Level 3 [Member] | Commitments [Member]
Liabilities:
Total liabilities measured at fair value	23,284	24,301
Level 3 [Member] | Accrued Liabilities [Member]
Liabilities:
Total liabilities measured at fair value	$ 0	$ 0