Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Thousands	Dec. 31, 2016 USD ($) securities	Dec. 31, 2015 USD ($) securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,063,374	$ 556,823
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(19,560)	(2,667)
Available-for-Sale, 12 months or more fair value	15,849	82,690
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(500)	(1,058)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,079,223	639,513
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(20,060)	(3,725)
Held-to-Maturity:
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	203,424	128,350
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8,879)	(4,649)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	22,817	79,015
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,285)	(1,320)
Held-to-maturity Securities, Fair Value	226,241	207,365
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (10,164)	$ (5,969)
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | securities	396	198
US Treasury Securities [Member]
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 598	$ 2,092
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(4)
Available-for-Sale, 12 months or more fair value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	598	2,092
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1)	(4)
State, county and municipal securities
Held-to-Maturity:
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	42,518	10,182
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(533)	(39)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,831	9,476
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(9)	(188)
Held-to-maturity Securities, Fair Value	45,349	19,658
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(542)	(227)
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	316,511	209,631
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(6,457)	(1,077)
Available-for-Sale, 12 months or more fair value	0	54,619
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(533)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	316,511	264,250
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(6,457)	(1,610)
Held-to-Maturity:
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	19,575
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(162)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held-to-maturity Securities, Fair Value	19,575
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(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	746,265	343,875
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(13,102)	(1,577)
Available-for-Sale, 12 months or more fair value	15,801	28,010
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(499)	(524)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	762,066	371,885
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(13,601)	(2,101)
Held-to-Maturity:
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	108,857	87,033
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(7,973)	(4,390)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	19,986	69,539
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,276)	(1,132)
Held-to-maturity Securities, Fair Value	128,843	156,572
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9,249)	(5,522)
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, Accumulated Loss	0	0
Available-for-Sale, 12 months or more fair value	48	61
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)	(1)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	48	61
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1)	(1)
Corporate securities
Held-to-Maturity:
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	32,474	31,135
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(211)	(220)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Held-to-maturity Securities, Fair Value	32,474	31,135
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(211)	(220)
Other Investments
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	1,225
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(9)
Available-for-Sale, 12 months or more fair value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	1,225
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 0	$ (9)