Investment Securities - Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment Securities Available-for-Sale: Amortized Cost
Due in one year or less	$ 47,145
Due after one year through five years	85,636
Due after five years through ten years	537,851
Due after ten years	2,640,878
Amortized Cost	3,311,510	$ 3,069,645
Investment Securities Available-for-Sale: Fair Value
Due in one year or less	47,088
Due after one year through five years	84,973
Due after five years through ten years	530,965
Due after ten years	2,638,028
Total	3,301,054	3,021,832
Investment Securities Held-to-Maturity: Amortized Cost
Due in one year or less	55,696
Due after one year through five years	229,594
Due after five years through ten years	162,812
Due after ten years	174,417
Amortized Cost	622,519	624,099
Investment Securities Held-to-Maturity: Fair Value
Due in one year or less	55,676
Due after one year through five years	230,314
Due after five years through ten years	165,238
Due after ten years	176,387
Total	627,615	618,672
Collateral Pledged
Investment Securities Held-to-Maturity: Amortized Cost
Amortized Cost	$ 971,100	$ 1,050,000