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	Mar. 31, 2018	Dec. 31, 2017
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 12,499	$ 5,830
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(183)	(26)
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	12,499	5,830
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(183)	(26)
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	163,345	11,650
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3,132)	(50)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	114,024	118,297
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(6,648)	(2,499)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	277,369	129,947
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9,780)	(2,549)
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	9,466	4,544
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(130)	(48)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	28,219	32,163
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,764)	(1,828)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	37,685	36,707
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,894)	(1,876)
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	456,200	303,932
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(7,184)	(2,408)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	176,076	187,690
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(9,192)	(7,387)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	632,276	491,622
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(16,376)	(9,795)
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	527,502	511,378
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(13,611)	(4,348)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	471,703	500,375
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(19,811)	(10,893)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	999,205	1,011,753
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(33,422)	(15,241)
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	196,887	190,985
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3,884)	(2,118)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	21,539	21,770
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(617)	(478)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	218,426	212,755
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4,501)	(2,596)
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	103,049	98,668
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4,361)	(8,742)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	103,049	98,668
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4,361)	(8,742)
Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,365,899	1,028,319
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(28,124)	(8,998)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	914,610	958,963
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(42,393)	(31,827)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,280,509	1,987,282
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (70,517)	$ (40,825)