Notes Payable, Collateralized and Short-Term Borrowings - Carrying Amounts and Estimated Fair Values of Company's Senior Notes (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	$ 948,257	$ 949,557
Estimated fair value of the Company's long-term debt	1,001,010	993,600
8.125% Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	109,334	109,271
Estimated fair value of the Company's long-term debt	115,335	115,650
5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	297,573	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	244,750	246,988
Estimated fair value of the Company's long-term debt	253,800	256,650
5.125% Senior Notes [Member]
Debt Instrument [Line Items]
Carrying amount of the Company's long-term debt	296,600	296,215
Estimated fair value of the Company's long-term debt	$ 316,125	$ 309,300