Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Investment Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		$ 3,000
Due within one year, Amortized Cost		206,812
Available-for-Sale Securities, Debt Maturities, Single Maturity Date, Fair Value		201,342
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	837,118
Due after one but within five years, Amortized Cost		132,852
Due after five but within ten years, Amortized Cost		25,657
Due after ten years, Amortized Cost		45,303
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	847,471
Amortized Cost		1,045,850	$ 1,317,318
Due within one year, Estimated Fair Value		3,000
Due after one but within five years, Estimated Fair Value		132,781
Due after five but within ten years, Estimated Fair Value		26,031
Due after ten years, Estimated Fair Value		39,530
Total Debt Securities, Estimated Fair Value		1,050,733	1,309,819
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost		1,043,930	1,315,898
Total Debt Securities, Estimated Fair Value		1,048,813	1,308,399
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost		20,902	23,344
Total Debt Securities, Estimated Fair Value		23,281	$ 25,936
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost		816,200
Total Debt Securities, Estimated Fair Value		$ 824,200

[1] Mortgage Backed Securities include an amortized cost of $20.9 million and a fair value of $23.3 million for Obligations of U.S.