Securities - Securities with Unrealized Losses - Securities Held-to-maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Held-to-maturity, Fair Value
Less Than 12 Months	$ 21,107	$ 186,598
12 Months or More	313,218	149,999
Total	334,325	336,597
Held-to-maturity, Unrealized (Losses)
Less Than 12 Months	(130)	(1,550)
12 Months or More	(11,264)	(3,451)
Total	(11,394)	(5,001)
Agency mortgage-backed securities
Held-to-maturity, Fair Value
Less Than 12 Months	16,088	168,439
12 Months or More	255,917	130,759
Total	272,005	299,198
Held-to-maturity, Unrealized (Losses)
Less Than 12 Months	(103)	(1,368)
12 Months or More	(9,050)	(3,026)
Total	(9,153)	(4,394)
Municipals - exempt from Federal tax
Held-to-maturity, Fair Value
Less Than 12 Months	5,019	18,159
12 Months or More	57,301	19,240
Total	62,320	37,399
Held-to-maturity, Unrealized (Losses)
Less Than 12 Months	(27)	(182)
12 Months or More	(2,214)	(425)
Total	$ (2,241)	$ (607)