Investment Securities - (Summary of Held-to-Maturity Securities, Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Fair Value, Less than 12 months	$ 177,035	$ 155,642
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,198)	(285)
Fair Value, 12 months or more	59,552	63,770
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,468)	(1,825)
Total Fair Value	236,587	219,412
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4,666)	(2,110)
Residential Mortgage Pass-Through Securities Issued or Guaranteed by U.S. Government Agencies or Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Fair Value, Less than 12 months	171,017	149,182
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,059)	(220)
Fair Value, 12 months or more	16,202	17,506
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(476)	(235)
Total Fair Value	187,219	166,688
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2,535)	(455)
Other Residential MBS Issued or Guaranteed by U.S. Government Agencies or Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Fair Value, Less than 12 months	6,018	6,460
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(139)	(65)
Fair Value, 12 months or more	43,350	46,264
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,992)	(1,590)
Total Fair Value	49,368	52,724
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (2,131)	$ (1,655)