Long-term Debt, Collateralized and Short-term Borrowings - Carrying Amounts and Estimated Fair Values of Company's Senior Notes (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	$ 947,592	$ 949,557
Estimated fair value of the Company's long-term debt	1,001,550	993,600
8.125% Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	109,365	109,271
Estimated fair value of the Company's long-term debt	117,000	115,650
5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	297,819	297,083
Estimated fair value of the Company's long-term debt	315,750	312,000
8.375% Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	243,611	246,988
Estimated fair value of the Company's long-term debt	250,800	256,650
5.125% Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	296,797	296,215
Estimated fair value of the Company's long-term debt	$ 318,000	$ 309,300