Regulatory Matters (Information On The Capital Ratios) (Detail)	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 2,519,589,000		$ 2,516,361,000
Common Equity Tier One Capital Ratio	0.1123		0.1156
Common Equity Tier One Capital Required for Capital Adequacy	$ 1,009,651,000		$ 979,739,000
Common Equity Tier One Capital Required to be Well-Capitalized	1,458,385,000		1,415,179,000
Total risk-based capital, Actual Amount	$ 3,022,511,000		$ 2,950,181,000
Total risk-based capital, Actual Ratio	0.1347		0.1355
Total risk-based capital, Capital Requirements, Minimum Amount	$ 1,794,935,000		$ 1,741,758,000
Total risk-based capital, Capital Requirements, Minimum Ratio	0.080		0.080
Total risk-based capital, Capital Requirements, Well Capitalized Amount	$ 2,243,669,000		$ 2,177,198,000
Total risk-based capital, Capital Requirements, Well Capitalized Ratio	0.100		0.100
Tier 1 capital, Actual Amount	$ 2,664,626,000		$ 2,661,398,000
Tier 1 capital, Actual Ratio	0.1188		0.1222
Tier 1 capital, Capital Requirements, Minimum Amount	$ 1,346,201,000		$ 1,306,319,000
Tier 1 capital, Capital Requirements, Minimum Ratio	0.060		0.060
Tier 1 capital, Capital Requirements, Well Capitalized Amount	$ 1,794,935,000		$ 1,741,758,000
Tier 1 capital, Capital Requirements, Well Capitalized Ratio	0.080		0.080
Tier 1 leverage capital ratio, Actual Amount	$ 2,664,626,000		$ 2,661,398,000
Tier 1 leverage capital ratio, Actual Ratio	0.0824		0.0896
Tier 1 leverage capital ratio, Capital Requirements, Minimum Amount	$ 1,293,186,000		$ 1,188,507,000
Tier 1 leverage capital ratio, Capital Requirements, Minimum Ratio	0.040		0.040
Tier 1 leverage capital ratio, Capital Requirements, Well Capitalized Amount	$ 1,616,482,000		$ 1,485,634,000
Tier 1 leverage capital ratio, Capital Requirements, Well Capitalized Ratio	0.050		0.050
Dividends paid	$ 0	$ 170,000,000
Subsidiaries [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 2,744,235,000		$ 2,527,645,000
Common Equity Tier One Capital Ratio	0.1224		0.1161
Common Equity Tier One Capital Required for Capital Adequacy	$ 1,009,277,000		$ 979,497,000
Common Equity Tier One Capital Required to be Well-Capitalized	1,457,845,000		1,414,829,000
Total risk-based capital, Actual Amount	$ 3,024,694,000		$ 2,739,108,000
Total risk-based capital, Actual Ratio	0.1349		0.1258
Total risk-based capital, Capital Requirements, Minimum Amount	$ 1,794,270,000		$ 1,741,328,000
Total risk-based capital, Capital Requirements, Minimum Ratio	0.080		0.080
Total risk-based capital, Capital Requirements, Well Capitalized Amount	$ 2,242,838,000		$ 2,176,660,000
Total risk-based capital, Capital Requirements, Well Capitalized Ratio	0.100		0.100
Tier 1 capital, Actual Amount	$ 2,744,235,000		$ 2,527,645,000
Tier 1 capital, Actual Ratio	0.1224		0.1161
Tier 1 capital, Capital Requirements, Minimum Amount	$ 1,345,703,000		$ 1,305,996,000
Tier 1 capital, Capital Requirements, Minimum Ratio	0.060		0.060
Tier 1 capital, Capital Requirements, Well Capitalized Amount	$ 1,794,270,000		$ 1,741,328,000
Tier 1 capital, Capital Requirements, Well Capitalized Ratio	0.080		0.080
Tier 1 leverage capital ratio, Actual Amount	$ 2,744,235,000		$ 2,527,645,000
Tier 1 leverage capital ratio, Actual Ratio	0.0849		0.0851
Tier 1 leverage capital ratio, Capital Requirements, Minimum Amount	$ 1,292,763,000		$ 1,187,953,000
Tier 1 leverage capital ratio, Capital Requirements, Minimum Ratio	0.040		0.040
Tier 1 leverage capital ratio, Capital Requirements, Well Capitalized Amount	$ 1,615,954,000		$ 1,484,941,000
Tier 1 leverage capital ratio, Capital Requirements, Well Capitalized Ratio	0.050		0.050
Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital Ratio	0.045		0.045
Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets [Member] | Subsidiaries [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital Ratio	0.045		0.045
Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital Ratio	0.065		0.065
Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets [Member] | Subsidiaries [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital Ratio	0.065		0.065