Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 216,533	$ 1,163,715
Less Than 12 Months, Unrealized Losses	1,703	23,697
12 Months or Longer, Fair Value	323,302	80,362
12 Months or Longer, Unrealized Losses	5,378	2,162
Total, Fair Value	539,835	1,244,077
Total, Unrealized Losses	7,081	25,859
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	22,414	232,535
Less Than 12 Months, Unrealized Losses	25	5,043
12 Months or Longer, Fair Value	14,776
12 Months or Longer, Unrealized Losses	224
Total, Fair Value	37,190	232,535
Total, Unrealized Losses	249	5,043
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	194,119	931,180
Less Than 12 Months, Unrealized Losses	1,678	18,654
12 Months or Longer, Fair Value	308,526	80,362
12 Months or Longer, Unrealized Losses	5,154	2,162
Total, Fair Value	502,645	1,011,542
Total, Unrealized Losses	$ 6,832	$ 20,816