Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment securities available for sale
Total investment securities available for sale, cost	$ 7,052,152	$ 4,607,117
Investment securities available for sale	7,059,674	4,557,110
Investment securities held to maturity
Maturing in one year or less, cost	30,746	0
Maturing in one year or less, fair value	30,746	0
Maturing in one through five years, cost	250	0
Maturing in one through five years, fair value	250	0
Investment securities held to maturity, cost	30,996	2,184,653
Total investment securities held to maturity, fair value	30,996	2,201,502
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	406,325	1,049,253
Maturing in one through five years, cost	24,496	205,526
Maturing in five through ten years, cost	185,209	134,370
Maturing in over ten years, cost	109,872	3,923
Maturing in one year or less, fair value	406,927	1,047,380
Maturing in one through five years, fair value	24,971	205,805
Maturing in five through ten years, fair value	187,868	133,626
Maturing in over ten years, fair value	110,026	4,125
Government Agency
Investment securities available for sale
Total investment securities available for sale, cost	684,085	257,252
Investment securities available for sale	682,772	256,835
Residential Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	5,269,060	2,956,793
Investment securities available for sale	5,267,090	2,909,339
Investment securities held to maturity
Investment securities held to maturity, cost	0	2,087,024
Investment securities held to maturity, fair value	0	2,103,126
Total investment securities held to maturity, fair value		2,103,126
Commercial Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	373,105	0
Investment securities available for sale	380,020	0
Investment securities held to maturity
Investment securities held to maturity, cost	0	97,629
Investment securities held to maturity, fair value	$ 0	98,376
Total investment securities held to maturity, fair value		$ 98,376