Debt Securities Available for Sale and Debt Securities Held to Maturity - Amortized Cost and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized cost, debt securities available for sale
Due in one year or less	$ 0	$ 6
Due in 1-5 years	24,464	15,178
Due in 5-10 years	509,832	514,336
Due after 10 years	1,962,708	1,188,834
Amortized Cost	2,497,004	1,718,354
Fair value, debt securities available for sale
Due in one year or less	0	6
Due in 1-5 years	24,375	15,312
Due in 5-10 years	503,768	517,867
Due after 10 years	1,941,922	1,175,833
Fair Value	2,470,065	1,709,018
Amortized cost, debt securities held to maturity
Due in one year or less	0	1,000
Due in 1-5 years	2,400	1,400
Due in 5-10 years	430	400
Due after 10 years	146,022	158,776
Amortized Cost	148,852	161,576
Fair value, debt securities held to maturity
Due in one year or less	0	1,000
Due in 1-5 years	2,400	1,400
Due in 5-10 years	432	400
Due after 10	146,435	162,826
Fair Value	$ 149,267	$ 165,626