Regulatory Matters (Information On The Capital Ratios) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 2,093,116	$ 1,932,171
Common Equity Tier One Capital Required for Capital Adequacy	$ 845,389	$ 826,504
Common Equity Tier One Capital Ratio	11.14%	10.52%
Common Equity Tier One Capital Required to be Well-Capitalized	$ 1,221,118	$ 1,193,840
Total risk-based capital, Actual Amount	$ 2,517,848	$ 2,328,808
Total risk-based capital, Actual Ratio	13.40%	12.68%
Total risk-based capital, Capital Requirements, Minimum Amount	$ 1,502,914	$ 1,469,341
Total risk-based capital, Capital Requirements, Minimum Ratio	8.00%	8.00%
Total risk-based capital, Capital Requirements, Well Capitalized Amount	$ 1,878,643	$ 1,836,677
Total risk-based capital, Capital Requirements, Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital, Actual Amount	$ 2,238,172	$ 2,054,881
Tier 1 risk-based capital, Actual Ratio	11.91%	11.19%
Tier 1 risk-based capital, Capital Requirements, Minimum Amount	$ 1,127,186	$ 1,102,006
Tier 1 risk-based capital, Capital Requirements, Minimum Ratio	6.00%	6.00%
Tier 1 risk-based capital, Capital Requirements, Well Capitalized Amount	$ 1,502,914	$ 1,469,341
Tier 1 risk-based capital, Capital Requirements, Well Capitalized Ratio	8.00%	8.00%
Tier 1 leverage capital, Actual Amount	$ 2,238,172	$ 2,054,881
Tier 1 leverage capital, Actual Ratio	8.63%	8.13%
Tier 1 leverage capital, Capital Requirements, Minimum Amount	$ 1,036,817	$ 1,010,857
Tier 1 leverage capital, Capital Requirements, Minimum Ratio	4.00%	4.00%
Tier 1 leverage capital, Capital Requirements, Well Capitalized Amount	$ 1,296,021	$ 1,263,571
Tier 1 leverage capital, Capital Requirements, Well Capitalized Ratio	5.00%	5.00%
Subsidiaries [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 2,114,224	$ 1,945,332
Common Equity Tier One Capital Required for Capital Adequacy	$ 844,693	$ 825,228
Common Equity Tier One Capital Ratio	11.26%	10.61%
Common Equity Tier One Capital Required to be Well-Capitalized	$ 1,220,113	$ 1,191,995
Total risk-based capital, Actual Amount	$ 2,316,580	$ 2,141,939
Total risk-based capital, Actual Ratio	12.34%	11.68%
Total risk-based capital, Capital Requirements, Minimum Amount	$ 1,501,677	$ 1,467,071
Total risk-based capital, Capital Requirements, Minimum Ratio	8.00%	8.00%
Total risk-based capital, Capital Requirements, Well Capitalized Amount	$ 1,877,097	$ 1,833,839
Total risk-based capital, Capital Requirements, Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital, Actual Amount	$ 2,114,224	$ 1,945,332
Tier 1 risk-based capital, Actual Ratio	11.26%	10.61%
Tier 1 risk-based capital, Capital Requirements, Minimum Amount	$ 1,126,258	$ 1,100,304
Tier 1 risk-based capital, Capital Requirements, Minimum Ratio	6.00%	6.00%
Tier 1 risk-based capital, Capital Requirements, Well Capitalized Amount	$ 1,501,677	$ 1,467,071
Tier 1 risk-based capital, Capital Requirements, Well Capitalized Ratio	8.00%	8.00%
Tier 1 leverage capital, Actual Amount	$ 2,114,224	$ 1,945,332
Tier 1 leverage capital, Actual Ratio	8.14%	7.70%
Tier 1 leverage capital, Capital Requirements, Minimum Amount	$ 1,038,442	$ 1,010,005
Tier 1 leverage capital, Capital Requirements, Minimum Ratio	4.00%	4.00%
Tier 1 leverage capital, Capital Requirements, Well Capitalized Amount	$ 1,298,052	$ 1,262,507
Tier 1 leverage capital, Capital Requirements, Well Capitalized Ratio	5.00%	5.00%
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	4.50%	4.50%
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	4.50%	4.50%
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	6.50%	6.50%
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	6.50%	6.50%