Securities - Amortized Cost and Fair Value of Held to Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	$ 2,250,960	$ 2,611,352
Securities Held to Maturity, Gross Unrealized Gains	21,225	2,593
Securities Held to Maturity, Gross Unrealized Losses	18,966	37,361
Securities Held to Maturity, Fair Value	2,253,219	2,576,584
U.S. 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		316
Securities Held to Maturity, Gross Unrealized Losses
Securities Held to Maturity, Fair Value		100,316
Municipal obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	183,379	193,189
Securities Held to Maturity, Gross Unrealized Gains	2,911	919
Securities Held to Maturity, Gross Unrealized Losses	1,709	6,436
Securities Held to Maturity, Fair Value	184,581	187,672
Mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	926,163	1,003,327
Securities Held to Maturity, Gross Unrealized Gains	12,426	296
Securities Held to Maturity, Gross Unrealized Losses	1,258	4,671
Securities Held to Maturity, Fair Value	937,331	998,952
Collateralized mortgage obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities Held to Maturity, Amortized Cost	1,141,418	1,314,836
Securities Held to Maturity, Gross Unrealized Gains	5,888	1,062
Securities Held to Maturity, Gross Unrealized Losses	15,999	26,254
Securities Held to Maturity, Fair Value	$ 1,131,307	$ 1,289,644