Investment Securities (Summary Of Available For Sale Investment Securities By Maturity Groupings) (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 9,961,155,000	$ 8,435,531,000
Debt Securities, Available-for-sale, Fair Value	10,317,427,000	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	769,463,000	827,861,000
Debt Securities, Available-for-sale, Fair Value	826,494,000	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	58,781,000
Debt Securities, Available-for-sale, Fair Value	59,246,000
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	487,146,000
Debt Securities, Available-for-sale, Fair Value	521,029,000
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	223,536,000
Debt Securities, Available-for-sale, Fair Value	246,219,000
Government-Sponsored Enterprise Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	106,019,000	138,734,000
Debt Securities, Available-for-sale, Fair Value	109,883,000	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	70,193,000
Debt Securities, Available-for-sale, Fair Value	70,308,000
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	35,826,000
Debt Securities, Available-for-sale, Fair Value	39,575,000
State and Municipal Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,385,113,000	1,225,532,000
Debt Securities, Available-for-sale, Fair Value	1,448,952,000	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	52,876,000
Debt Securities, Available-for-sale, Fair Value	53,255,000
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	753,093,000
Debt Securities, Available-for-sale, Fair Value	788,553,000
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	425,285,000
Debt Securities, Available-for-sale, Fair Value	450,376,000
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	153,859,000
Debt Securities, Available-for-sale, Fair Value	156,768,000
Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	5,205,895,000	3,893,247,000
Debt Securities, Available-for-sale, Fair Value	5,382,187,000	3,937,964,000
Non-Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	581,257,000	796,451,000
Debt Securities, Available-for-sale, Fair Value	600,196,000	809,782,000
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,431,854,000	1,228,151,000
Debt Securities, Available-for-sale, Fair Value	1,448,225,000	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	7,219,006,000	5,917,849,000
Debt Securities, Available-for-sale, Fair Value	7,430,608,000	5,981,235,000
Other Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	481,554,000	325,555,000
Debt Securities, Available-for-sale, Fair Value	501,490,000	$ 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	31,933,000
Debt Securities, Available-for-sale, Fair Value	32,056,000
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	248,497,000
Debt Securities, Available-for-sale, Fair Value	260,309,000
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	166,931,000
Debt Securities, Available-for-sale, Fair Value	173,677,000
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	34,193,000
Debt Securities, Available-for-sale, Fair Value	$ 35,448,000