Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Available-for-Sale:
Available-for-Sale, Amortized Cost	$ 1,966,417	$ 1,983,629
Available-for-Sale, Gross Unrealized Gains	30,531	33,927
Available-for-Sale, Gross Unrealized Losses	(1,690)	(692)
Available-for-Sale, Estimated Fair Value	1,995,258	2,016,864
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	208,223	152,781
Held-to-Maturity, Gross Unrealized Gains	10,899	9,113
Held-to-Maturity, Gross Unrealized Losses	(189)	(17)
Held-to-Maturity, Estimated Fair Value	218,933	161,877
Gross realized gains	351	1,544	173
Gross realized losses	3	0	3
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	751,501	1,134,427
Available-for-Sale, Gross Unrealized Gains	3,518	4,353
Available-for-Sale, Gross Unrealized Losses	(163)	(662)
Available-for-Sale, Estimated Fair Value	754,856	1,138,118
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,214,377	848,444
Available-for-Sale, Gross Unrealized Gains	27,000	29,567
Available-for-Sale, Gross Unrealized Losses	(1,526)	(14)
Available-for-Sale, Estimated Fair Value	1,239,851	877,997
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	539	758
Available-for-Sale, Gross Unrealized Gains	13	7
Available-for-Sale, Gross Unrealized Losses	(1)	(16)
Available-for-Sale, Estimated Fair Value	551	749
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	192,875	152,619
Held-to-Maturity, Gross Unrealized Gains	10,835	9,113
Held-to-Maturity, Gross Unrealized Losses	(176)	(17)
Held-to-Maturity, Estimated Fair Value	203,534	161,715
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	14,975
Held-to-Maturity, Gross Unrealized Gains	64
Held-to-Maturity, Gross Unrealized Losses	(13)
Held-to-Maturity, Estimated Fair Value	15,026
Other securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	373	162
Held-to-Maturity, Gross Unrealized Gains	0	0
Held-to-Maturity, Gross Unrealized Losses	0	0
Held-to-Maturity, Estimated Fair Value	$ 373	$ 162