Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Investment securities available for sale
Total investment securities available for sale, cost	$ 8,472,398	$ 7,052,152
Total investment securities available for sale, fair value	8,619,282	7,059,674
Investment securities held to maturity
Investment securities held to maturity at cost	774,878	30,996
Investment securities held to maturity at fair value	791,325	30,996
Investment securities available for sale at cost	8,472,398	7,052,152
Debt Securities, Available-for-sale	8,619,282	7,059,674
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	2,257	30,746
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	2,257	30,746
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	749	250
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	749	250
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	754,350	406,325
Maturing in one through five years, cost	51,589	24,496
Maturing in five through ten years, cost	369,075	185,209
Maturing in over ten years, cost	28,959	109,872
Maturing in one year or less, fair value	754,586	406,927
Maturing in one through five years, fair value	51,887	24,971
Maturing in five through ten yeas, fair value	368,281	187,868
Maturing in over ten years, fair value	28,605	110,026
Agency Securities
Investment securities held to maturity
Investment securities available for sale at cost	549,673	684,085
Debt Securities, Available-for-sale	545,709	682,772
Residential Mortgage Backed Securities
Investment securities held to maturity
Investment securities held to maturity at cost	693,143	0
Commercial mortgage-backed securities	709,339	0
Investment securities held to maturity at fair value	709,339
Investment securities available for sale at cost	5,671,151	5,269,060
Debt Securities, Available-for-sale	5,790,392	5,267,090
Commercial Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	1,047,601	373,105
Total investment securities available for sale, fair value	1,079,822	380,020
Investment securities held to maturity
Investment securities held to maturity at cost	78,729	0
Commercial mortgage-backed securities	78,980	0
Investment securities held to maturity at fair value	$ 78,980
Residential mortgage-backed securities
Investment securities held to maturity
Investment securities held to maturity at cost		30,996
Investment securities held to maturity at fair value		$ 30,996