Investment Securities - Amortized Cost and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 60,739	$ 0
Due in 1-5 years	199,777	24,464
Due in 5-10 years	483,336	509,832
Due after 10 years	4,750,770	1,962,708
Amortized Cost	5,494,622	2,497,004
Fair value
Due in one year or less	60,652	0
Due in 1-5 years	199,399	24,375
Due in 5-10 years	492,755	503,768
Due after 10 years	4,827,029	1,941,922
Total investment securities available-for-sale	5,579,835	2,470,065
Amortized cost
Due in 1-5 years	3,150	2,400
Due in 5-10 years	460	430
Due after 10 years	140,390	146,022
Amortized Cost	144,000	148,852
Fair value
Due in 1-5 years	3,150	2,400
Due in 5-10 years	467	432
Due after 10 years	146,311	146,435
Fair Value	$ 149,928	$ 149,267