Investment Securities Gross Unrealized Losses and Fair Values of Investments by Category and Length of Time in a Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 121,527	$ 247,509
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,930)	(15,676)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	87,466	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3,475)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	208,993	247,509
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5,405)	(15,676)
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,570	11,922
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(16)	(110)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	31,533	34,629
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,940)	(4,475)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	35,103	46,551
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,956)	(4,585)
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	85,335	166,905
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(837)	(3,899)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	301,009	258,237
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8,710)	(8,279)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	386,344	425,142
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9,547)	(12,178)
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	796,019	1,112,947
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8,359)	(16,900)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,513	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(202)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	801,532	1,112,947
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(8,561)	(16,900)
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	87,260	24,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(627)	(537)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, 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	87,260	24,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(627)	(537)
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
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 Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	98,156	97,256
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(9,254)	(9,959)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	98,156	97,256
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9,254)	(9,959)
Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,093,711	1,563,846
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(11,769)	(37,122)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	523,677	390,122
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(23,581)	(22,713)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,617,388	1,953,968
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (35,350)	$ (59,835)