Securities - Amortized Cost and Fair Value of Held to Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	$ 2,666,082	$ 1,668,018
Securities Held to Maturity, Gross Unrealized Gains	13,713	43,388
Securities Held to Maturity, Gross Unrealized Losses	25,928	941
Securities Held to Maturity, Fair Value	2,653,867	1,710,465
US Treasury and government agency securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	100,000
Securities Held to Maturity, Gross Unrealized Gains	688
Securities Held to Maturity, Fair Value	100,688
Municipal obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	195,662	164,493
Securities Held to Maturity, Gross Unrealized Gains	1,137	16,017
Securities Held to Maturity, Gross Unrealized Losses	5,291
Securities Held to Maturity, Fair Value	191,508	180,510
Mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	995,686	180,397
Securities Held to Maturity, Gross Unrealized Gains	9,273	3,429
Securities Held to Maturity, Gross Unrealized Losses	2,118
Securities Held to Maturity, Fair Value	1,002,841	183,826
Collateralized mortgage obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	1,374,734	1,323,128
Securities Held to Maturity, Gross Unrealized Gains	2,615	23,942
Securities Held to Maturity, Gross Unrealized Losses	18,519	941
Securities Held to Maturity, Fair Value	$ 1,358,830	$ 1,346,129