Investment Securities (Summary Of Available For Sale Investment Securities By Maturity Groupings) (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 8,437,595,000	$ 8,435,531,000
Debt Securities, Available-for-sale, Fair Value	8,678,586,000	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	799,200,000	827,861,000
Debt Securities, Available-for-sale, Fair Value	840,447,000	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	86,714,000
Debt Securities, Available-for-sale, Fair Value	86,079,000
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	487,114,000
Debt Securities, Available-for-sale, Fair Value	517,959,000
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	225,372,000
Debt Securities, Available-for-sale, Fair Value	236,409,000
Government-Sponsored Enterprise Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	107,841,000	138,734,000
Debt Securities, Available-for-sale, Fair Value	112,870,000	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	70,167,000
Debt Securities, Available-for-sale, Fair Value	70,749,000
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	37,674,000
Debt Securities, Available-for-sale, Fair Value	42,121,000
State and Municipal Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,247,621,000	1,225,532,000
Debt Securities, Available-for-sale, Fair Value	1,283,068,000	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	55,673,000
Debt Securities, Available-for-sale, Fair Value	55,911,000
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	757,906,000
Debt Securities, Available-for-sale, Fair Value	778,801,000
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	366,186,000
Debt Securities, Available-for-sale, Fair Value	380,949,000
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	67,856,000
Debt Securities, Available-for-sale, Fair Value	67,407,000
Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	4,011,631,000	3,893,247,000
Debt Securities, Available-for-sale, Fair Value	4,168,421,000	3,937,964,000
Non-Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	708,554,000	796,451,000
Debt Securities, Available-for-sale, Fair Value	717,008,000	809,782,000
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,243,235,000	1,228,151,000
Debt Securities, Available-for-sale, Fair Value	1,232,292,000	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	5,963,420,000	5,917,849,000
Debt Securities, Available-for-sale, Fair Value	6,117,721,000	5,981,235,000
Other Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	319,513,000	325,555,000
Debt Securities, Available-for-sale, Fair Value	324,480,000	$ 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	41,966,000
Debt Securities, Available-for-sale, Fair Value	41,789,000
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	212,864,000
Debt Securities, Available-for-sale, Fair Value	215,466,000
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	61,691,000
Debt Securities, Available-for-sale, Fair Value	64,123,000
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	2,992,000
Debt Securities, Available-for-sale, Fair Value	$ 3,102,000