Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Thousands	Jun. 30, 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	$ 970,913	$ 1,063,374
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(7,461)	(19,560)
Available-for-Sale, 12 Months or More Fair Value	31,879	15,849
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(430)	(500)
Available-for-Sale, Total Fair Value	1,002,792	1,079,223
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(7,891)	(20,060)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	88,499	203,424
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(8,252)	(8,879)
Held-to-Maturity, 12 Months or More Fair Value	18,650	22,817
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(1,293)	(1,285)
Held-to-Maturity, Total Fair Value	107,149	226,241
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (9,545)	$ (10,164)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities) | securities	337	396
US Treasury Securities [Member]
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,797	$ 598
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(2)	(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	1,797	598
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2)	(1)
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	359,889	316,511
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(2,804)	(6,457)
Available-for-Sale, 12 Months or More Fair Value	9,989	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(10)	0
Available-for-Sale, Total Fair Value	369,878	316,511
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2,814)	(6,457)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	9,985	19,575
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(5)	(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	9,985	19,575
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(5)	(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	599,643	746,265
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(4,346)	(13,102)
Available-for-Sale, 12 Months or More Fair Value	21,848	15,801
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(419)	(499)
Available-for-Sale, Total Fair Value	621,491	762,066
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(4,765)	(13,601)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	42,109	108,857
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(8,069)	(7,973)
Held-to-Maturity, 12 Months or More Fair Value	17,766	19,986
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(1,291)	(1,276)
Held-to-Maturity, Total Fair Value	59,875	128,843
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(9,360)	(9,249)
Private mortgage-backed securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,484	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(303)	0
Available-for-Sale, 12 Months or More Fair Value	42	48
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(1)	(1)
Available-for-Sale, Total Fair Value	1,526	48
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(304)	(1)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	21,331	42,518
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(139)	(533)
Held-to-Maturity, 12 Months or More Fair Value	884	2,831
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(2)	(9)
Held-to-Maturity, Total Fair Value	22,215	45,349
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(141)	(542)
Corporate securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	8,100
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(6)
Available-for-Sale, 12 Months or More Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	0
Available-for-Sale, Total Fair Value	8,100
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(6)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	15,074	32,474
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(39)	(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	15,074	32,474
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (39)	$ (211)