Securities - Remaining Contractual Maturity of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Amortized Cost [Abstract]
One year or less	$ 97,905
After one year to five years	303,899
After five years to ten years	0
After ten years	45,851
Securities that do not have a single contractual maturity date	1,315,866
Amortized Cost	1,763,521	$ 1,957,584
Available-for-sale Securities, Fair Value [Abstract]
One year or less	94,691
After one year to five years	293,924
After five years to ten years	0
After ten years	44,346
Securities that do not have a single contractual maturity date	1,300,023
Fair Value	1,732,984	1,919,450
Held-to-maturity Securities, Amortized Cost [Abstract]
One year or less	1,947
After one year to five years	6,065
After five years to ten years	2,243
After ten years	4,383
Securities that do not have a single contractual maturity date	0
Amortized Cost	14,638	$ 22,291
Held-to-maturity Securities, Fair Value [Abstract]
One year or less	1,718
After one year to five years	5,354
After five years to ten years	1,980
After ten years	3,869
Securities that do not have a single contractual maturity date	0
Fair Value	$ 12,921