Securities - Securities with Unrealized Losses - Securities Held-to-maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Held-to-maturity, Fair Value
Less Than 12 Months	$ 207,525	$ 279,436
12 Months or More	46,990	2,658
Total	254,515	282,094
Held-to-maturity, Unrealized Losses
Less Than 12 Months	(2,096)	(5,554)
12 Months or More	(922)	(244)
Total	(3,018)	(5,798)
Agency mortgage-backed securities
Held-to-maturity, Fair Value
Less Than 12 Months	191,603	222,132
12 Months or More	26,547	612
Total	218,150	222,744
Held-to-maturity, Unrealized Losses
Less Than 12 Months	(1,980)	(3,528)
12 Months or More	(361)	(26)
Total	(2,341)	(3,554)
Municipals - exempt from Federal tax
Held-to-maturity, Fair Value
Less Than 12 Months	15,922	57,304
12 Months or More	20,443	2,046
Total	36,365	59,350
Held-to-maturity, Unrealized Losses
Less Than 12 Months	(116)	(2,026)
12 Months or More	(561)	(218)
Total	$ (677)	$ (2,244)