Securities - Amortized Cost and Fair Value of Held-to-Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	$ 1,705,251	$ 1,668,018
Securities Held to Maturity, Gross Unrealized Gains	22,970	43,388
Securities Held to Maturity, Gross Unrealized Losses	16,505	941
Securities Held to Maturity, Fair Value	1,711,716	1,710,465
Municipal obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	195,863	164,493
Securities Held to Maturity, Gross Unrealized Gains	10,119	16,017
Securities Held to Maturity, Gross Unrealized Losses	3,243
Securities Held to Maturity, Fair Value	202,739	180,510
Mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	156,790	180,397
Securities Held to Maturity, Gross Unrealized Gains		3,429
Securities Held to Maturity, Gross Unrealized Losses	2,369
Securities Held to Maturity, Fair Value	154,421	183,826
Collateralized mortgage obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	1,352,598	1,323,128
Securities Held to Maturity, Gross Unrealized Gains	12,851	23,942
Securities Held to Maturity, Gross Unrealized Losses	10,893	941
Securities Held to Maturity, Fair Value	$ 1,354,556	$ 1,346,129