Securities (Unrealized Gross Losses and Estimated Fair values of Investment Securities by Length of Time that Individual Securities in Each Category in Continuous Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	$ 324,078	$ 511,991
Unrealized Losses - Less Than 12 Months	(3,089)	(8,686)
Fair Value - 12 Months or More	398,360	103,708
Unrealized Losses - 12 Months or More	(11,172)	(3,850)
Fair Value	722,438	615,699
Unrealized Losses	(14,261)	(12,536)
Available-for-sale Securities, Amortized Cost Basis	803,021	789,229
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	1,139	3,174
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	9,317	42,805
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(57)	(631)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,436	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(180)	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	18,753	42,805
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(237)	(631)
Obligations of U.S. government sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	221,466	348,579
Unrealized Losses - Less Than 12 Months	(2,393)	(5,780)
Fair Value - 12 Months or More	233,971	29,496
Unrealized Losses - 12 Months or More	(5,078)	(1,335)
Fair Value	455,437	378,075
Unrealized Losses	(7,471)	(7,115)
Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value - Less Than 12 Months	102,612	163,412
Unrealized Losses - Less Than 12 Months	(696)	(2,906)
Fair Value - 12 Months or More	164,389	74,212
Unrealized Losses - 12 Months or More	(6,094)	(2,515)
Fair Value	267,001	237,624
Unrealized Losses	(6,790)	(5,421)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	5,484	5,481
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	173	132
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	554	632
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	252	109
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	510,176	485,222
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	597	2,515
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	7,232	8,848
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	103	153
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	9,317
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(57)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(180)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	18,753
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (237)
US Government Corporations and Agencies Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		42,805
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(631)
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, Continuous Unrealized Loss Position, Fair Value		42,805
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ (631)