Investments (Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment securities available for sale:
Less than 12 months, fair value	$ 760,153	$ 2,778,014
Less than 12 months, unrealized losses	6,466	16,025
Longer than 12 months, fair value	351,841	332,953
Longer than 12 months, unrealized losses	2,445	1,794
Total fair value	1,111,994	3,110,967
Total unrealized losses	8,911	17,819
Government Agency
Investment securities available for sale:
Less than 12 months, fair value	268,622	347,081
Less than 12 months, unrealized losses	3,197	1,827
Longer than 12 months, fair value	328,777	63,947
Longer than 12 months, unrealized losses	2,376	414
Total fair value	597,399	411,028
Total unrealized losses	5,573	2,241
Residential Mortgage Backed Securities
Investment securities available for sale:
Less than 12 months, fair value	433,816	2,387,293
Less than 12 months, unrealized losses	1,241	14,016
Longer than 12 months, fair value	23,064	264,257
Longer than 12 months, unrealized losses	69	1,371
Total fair value	456,880	2,651,550
Total unrealized losses	1,310	15,387
Commercial Mortgage Backed Securities
Investment securities available for sale:
Less than 12 months, fair value		35,926
Less than 12 months, unrealized losses		59
Longer than 12 months, fair value		0
Longer than 12 months, unrealized losses		0
Total fair value		35,926
Total unrealized losses		59
Corporate Bonds
Investment securities available for sale:
Less than 12 months, fair value	57,715	7,714
Less than 12 months, unrealized losses	2,028	123
Longer than 12 months, fair value	0	4,749
Longer than 12 months, unrealized losses	0	9
Total fair value	57,715	12,463
Total unrealized losses	$ 2,028	$ 132