Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Jun. 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		206,551
Available-for-Sale Securities, Debt Maturities, Single Maturity Date, Fair Value		200,508
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	882,720
Due after one but within five years, Amortized Cost		132,852
Due after five but within ten years, Amortized Cost		25,655
Due after ten years, Amortized Cost		45,044
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	887,297
Amortized Cost		1,091,191	$ 1,317,318
Due within one year, Estimated Fair Value		2,999
Due after one but within five years, Estimated Fair Value		132,629
Due after five but within ten years, Estimated Fair Value		25,676
Due after ten years, Estimated Fair Value		39,204
Total Debt Securities, Estimated Fair Value		1,089,725	1,309,819
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost		1,089,271	1,315,898
Total Debt Securities, Estimated Fair Value		1,087,805	1,308,399
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost		21,846	23,344
Total Debt Securities, Estimated Fair Value		24,338	$ 25,936
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost		860,900
Total Debt Securities, Estimated Fair Value		$ 863,000

[1] Mortgage Backed Securities include an amortized cost of $21.8 million and a fair value of $24.3 million for Obligations of U.S.