Investment Securities (Summary Of Available For Sale Investment Securities By Maturity Groupings) (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 11,182,348,000	$ 8,435,531,000
Debt Securities, Available-for-sale, Fair Value	11,539,061,000	8,571,626,000
U.S. Government and Federal Agency Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	773,568,000	827,861,000
Debt Securities, Available-for-sale, Fair Value	835,058,000	851,776,000
U.S. Government and Federal Agency Obligations [Member] | Available For Sale Securities Maturity Within1 Year [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	59,367,000
Debt Securities, Available-for-sale, Fair Value	59,706,000
U.S. Government and Federal Agency Obligations [Member] | Available For Sale Securities Maturity After1 But Within5 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	489,042,000
Debt Securities, Available-for-sale, Fair Value	521,797,000
U.S. Government and Federal Agency Obligations [Member] | Available For Sale Securities Maturity After5 But Within10 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	225,159,000
Debt Securities, Available-for-sale, Fair Value	253,555,000
Government-Sponsored Enterprise Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	91,003,000	138,734,000
Debt Securities, Available-for-sale, Fair Value	95,301,000	139,277,000
Government-Sponsored Enterprise Obligations [Member] | Available For Sale Securities Maturity Within1 Year [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	55,185,000
Debt Securities, Available-for-sale, Fair Value	54,770,000
Government-Sponsored Enterprise Obligations [Member] | Available For Sale Maturity After10 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	35,818,000
Debt Securities, Available-for-sale, Fair Value	40,531,000
State and Municipal Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,895,269,000	1,225,532,000
Debt Securities, Available-for-sale, Fair Value	1,969,144,000	1,267,927,000
State and Municipal Obligations [Member] | Available For Sale Securities Maturity Within1 Year [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	68,787,000
Debt Securities, Available-for-sale, Fair Value	69,179,000
State and Municipal Obligations [Member] | Available For Sale Securities Maturity After1 But Within5 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	764,831,000
Debt Securities, Available-for-sale, Fair Value	803,168,000
State and Municipal Obligations [Member] | Available For Sale Securities Maturity After5 But Within10 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	597,276,000
Debt Securities, Available-for-sale, Fair Value	624,525,000
State and Municipal Obligations [Member] | Available For Sale Maturity After10 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	464,375,000
Debt Securities, Available-for-sale, Fair Value	472,272,000
Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	5,919,500,000	3,893,247,000
Debt Securities, Available-for-sale, Fair Value	6,081,802,000	3,937,964,000
Non-Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	324,266,000	796,451,000
Debt Securities, Available-for-sale, Fair Value	332,970,000	809,782,000
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,633,988,000	1,228,151,000
Debt Securities, Available-for-sale, Fair Value	1,658,579,000	1,233,489,000
Total Mortgage And Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	7,877,754,000	5,917,849,000
Debt Securities, Available-for-sale, Fair Value	8,073,351,000	5,981,235,000
Other Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	544,754,000	325,555,000
Debt Securities, Available-for-sale, Fair Value	566,207,000	$ 331,411,000
Other Debt Securities [Member] | Available For Sale Securities Maturity Within1 Year [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	23,565,000
Debt Securities, Available-for-sale, Fair Value	23,701,000
Other Debt Securities [Member] | Available For Sale Securities Maturity After1 But Within5 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	252,180,000
Debt Securities, Available-for-sale, Fair Value	264,096,000
Other Debt Securities [Member] | Available For Sale Securities Maturity After5 But Within10 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	240,814,000
Debt Securities, Available-for-sale, Fair Value	249,136,000
Other Debt Securities [Member] | Available For Sale Maturity After10 Years [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	28,195,000
Debt Securities, Available-for-sale, Fair Value	$ 29,274,000