Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Mortgage Backed Securities [Member]		Sep. 30, 2013 Mortgage Backed Securities [Member]		Sep. 30, 2014 US Government Agencies Residential Mortgage-backed Securities [Member]	Dec. 31, 2013 US Government Agencies Residential Mortgage-backed Securities [Member]	Sep. 30, 2014 Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Due within one year, Amortized Cost	$ 2,600
Due after one but within five years, Amortized Cost	266,577
Due after five but within ten years, Amortized Cost	11,224
Due after ten years, Amortized Cost	56,853
Debt securities gross, Amortized Cost	337,254
Corporate/Mortgage-Backed Securities, Amortized cost	1,348,063	1,350,466	1,009,389	[1]	1,007,226	[1]	21,780	22,639	987,600
Debt Securities, Amortized Cost	1,346,643
Due within one year, Estimated Fair Value	2,600
Due after one but within five years, Estimated Fair Value	265,117
Due after five but within ten years, Estimated Fair Value	11,373
Due after ten years, Estimated Fair Value	45,038
Debt securities gross, Estimated Fair Value	324,128
Total Debt Securities, Estimated Fair Value	$ 1,331,354

[1] Mortgage Backed Securities include an amortized cost of $21.8 million and a fair value of $24.4 million for Obligations of U.S.