Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Thousands	Mar. 31, 2017 USD ($) securities	Dec. 31, 2016 USD ($) securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 977,380	$ 1,063,374
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(12,053)	(19,560)
Available-for-Sale, 12 Months or More Fair Value	33,309	15,849
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(596)	(500)
Available-for-Sale, Total Fair Value	1,010,689	1,079,223
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(12,649)	(20,060)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	136,695	203,424
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(8,183)	(8,879)
Held-to-Maturity, 12 Months or More Fair Value	20,951	22,817
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(1,274)	(1,285)
Held-to-Maturity, Total Fair Value	157,646	226,241
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (9,457)	$ (10,164)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities) | securities	319	396
US Treasury Securities [Member]
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 598	$ 598
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(1)	(1)
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	598	598
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1)	(1)
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	324,693	316,511
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(4,287)	(6,457)
Available-for-Sale, 12 Months or More Fair Value	9,984	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(15)	0
Available-for-Sale, Total Fair Value	334,677	316,511
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(4,302)	(6,457)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	19,655	19,575
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(82)	(162)
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	19,655	19,575
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(82)	(162)
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	652,089	746,265
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(7,765)	(13,102)
Available-for-Sale, 12 Months or More Fair Value	23,282	15,801
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(580)	(499)
Available-for-Sale, Total Fair Value	675,371	762,066
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(8,345)	(13,601)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	77,626	108,857
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(7,816)	(7,973)
Held-to-Maturity, 12 Months or More Fair Value	18,509	19,986
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(1,270)	(1,276)
Held-to-Maturity, Total Fair Value	96,135	128,843
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(9,086)	(9,249)
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	43	48
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(1)	(1)
Available-for-Sale, Total Fair Value	43	48
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1)	(1)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	23,353	42,518
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(218)	(533)
Held-to-Maturity, 12 Months or More Fair Value	2,442	2,831
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(4)	(9)
Held-to-Maturity, Total Fair Value	25,795	45,349
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(222)	(542)
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	16,061	32,474
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(67)	(211)
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	16,061	32,474
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (67)	$ (211)