Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013 securities	Dec. 31, 2012 securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,184,937	$ 344,180
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	29,378	1,689
Available-for-Sale, 12 Months or More Fair Value	112	137
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	3	1
Available-for-Sale, Total Fair Value	1,185,049	344,317
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	29,381	1,690
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	64,624	28,701
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(2,062)	(181)
Held-to-Maturity, 12 Months or More Fair Value	504	557
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(27)	(8)
Held-to-Maturity, Total Fair Value	65,128	29,258
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2,089)	(189)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities)	212	69
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	442,307	93,982
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	11,559	163
Available-for-Sale, 12 Months or More Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	0	0
Available-for-Sale, Total Fair Value	442,307	93,982
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	11,559	163
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	742,630	250,198
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	17,819	1,526
Available-for-Sale, 12 Months or More Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	0	0
Available-for-Sale, Total Fair Value	742,630	250,198
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	17,819	1,526
Private mortgage-backed securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	0	0
Available-for-Sale, 12 Months or More Fair Value	112	137
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	3	1
Available-for-Sale, Total Fair Value	112	137
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	3	1
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	52,225	19,389
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(1,963)	(168)
Held-to-Maturity, 12 Months or More Fair Value	504	557
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(27)	(8)
Held-to-Maturity, Total Fair Value	52,729	19,946
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1,990)	(176)
Corporate Debt Securities [Member]
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	12,399	9,312
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(99)	(13)
Held-to-Maturity, 12 Months or More Fair Value	0	0
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	0	0
Held-to-Maturity, Total Fair Value	12,399	9,312
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (99)	$ (13)