Financial Instruments and Fair Value Disclosures - Assets and Liabilities Measured at Fair Value on Recurring and Nonrecurring Basis (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Assets:
Loss on assets	$ 71,268	$ 678,145
Carrying value of impaired assets			$ 45,100
Fair Value Measurements, Recurring [Member]
Assets:
Short-term investments	262,887		146,360
Mortgage-backed securities	4		35
Total assets	262,891		146,395
Fair Value Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Short-term investments	262,887		146,360
Total assets	262,887		146,360
Fair Value Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Mortgage-backed securities	4		35
Total assets	4		35
Nonrecurring Fair Value Measurements [Member]
Assets:
Loss on assets	71,268		678,145
Carrying value of impaired assets	2,000		69,153
Nonrecurring Fair Value Measurements [Member] | Level 3 [Member]
Assets:
Carrying value of impaired assets	$ 2,000		$ 69,153