Investment Securities - (Summary of Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months Fair Value	$ 374,860	$ 225,038
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5,490)	(1,244)
12 months or more, Fair Value	417,837	449,053
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(20,881)	(16,982)
Total, Fair Value	792,697	674,091
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(26,371)	(18,226)
Residential Mortgage Pass-Through Securities Issued or Guaranteed by U.S. Government Agencies or Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months Fair Value	99,713	62,178
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,579)	(408)
12 months or more, Fair Value	32,040	36,086
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(856)	(584)
Total, Fair Value	131,753	98,264
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2,435)	(992)
Other Residential MBS Issued or Guaranteed by U.S. Government Agencies or Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months Fair Value	274,644	162,346
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3,894)	(830)
12 months or more, Fair Value	385,797	412,967
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(20,025)	(16,398)
Total, Fair Value	660,441	575,313
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(23,919)	(17,228)
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months Fair Value	503	514
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(17)	(6)
Total, Fair Value	503	514
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (17)	$ (6)