Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Investment Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	$ 6,800
Due within one year, Amortized Cost	336,457
Available-for-Sale Securities, Debt Maturities, Single Maturity Date, Fair Value	326,634
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	928,538	[1]
Due after one but within five years, Amortized Cost	254,050
Due after five but within ten years, Amortized Cost	24,420
Due after ten years, Amortized Cost	51,187
Corporate/Mortgage-Backed Securities, Amortized cost	940,731	[1]
Amortized Cost	1,266,915		1,317,318
Due within one year, Estimated Fair Value	6,798
Due after one but within five years, Estimated Fair Value	253,901
Due after five but within ten years, Estimated Fair Value	24,869
Due after ten years, Estimated Fair Value	41,066
Total Debt Securities, Estimated Fair Value	1,269,285		1,309,819
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,264,995		1,315,898
Total Debt Securities, Estimated Fair Value	1,267,365		1,308,399
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	22,578		23,344
Total Debt Securities, Estimated Fair Value	25,413		25,936
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	906,000
Total Debt Securities, Estimated Fair Value	$ 915,300

[1] Mortgage Backed Securities include an amortized cost of $22.6 million and a fair value of $25.4 million for Obligations of U.S.