Actual Capital Amounts and Ratios for Commercial Bank in Comparison With Minimum Amounts and Ratios Required (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Common Equity, Tier 1 Amount
Total regulatory capital, Common Equity, Tier 1 Amount	$ 3,869,129	$ 3,748,231
Minimum for capital adequacy purposes, Common Equity, Tier 1 Amount	1,532,448	1,588,699
Excess, Common Equity, Tier 1 Amount	2,336,681	2,159,532
Total regulatory capital, Leverage Capital, Amount	$ 4,371,969	$ 3,748,231
Common Equity, Tier 1 Ratio
Total regulatory capital, Common Equity, Tier 1 Ratio	11.36%	10.62%
Minimum for capital adequacy purposes, Common Equity, Tier 1 Ratio	4.50%	4.50%
Excess, Common Equity, Tier 1 Ratio	6.86%	6.12%
Minimum for capital adequacy purposes, Leverage Capital, Amount	$ 1,826,141	$ 1,875,062
Leverage Capital, Amount
Total regulatory capital, Leverage Capital, Amount	4,371,969	3,748,231
Minimum for capital adequacy purposes, Leverage Capital, Amount	1,826,141	1,875,062
Excess, Leverage Capital, Amount	2,545,828	1,873,169
Excess, Leverage Capital, Amount	$ 2,545,828	$ 1,873,169
Leverage Capital, Ratio
Total regulatory capital, Leverage Capital, Ratio	9.58%	8.00%
Minimum for capital adequacy purposes, Leverage Capital, Ratio	4.00%	4.00%
Excess, Leverage Capital, Ratio	5.58%	4.00%
Total regulatory capital, Leverage Capital, Ratio	9.58%	8.00%
Risk-Based Capital, Tier 1 Amount
Total regulatory capital, Risk-Based Capital, Tier 1 Amount	$ 4,371,969	$ 3,748,231
Minimum for capital adequacy purposes, Risk-Based Capital, Tier 1 Amount	2,043,265	2,118,266
Excess, Risk-Based Capital, Tier 1 Amount	$ 2,328,704	$ 1,629,965
Minimum for capital adequacy purposes, Leverage Capital, Ratio	4.00%	4.00%
Risk-Based Capital, Tier 1 Ratio
Total regulatory capital, Risk-Based Capital, Tier 1 Ratio	12.84%	10.62%
Minimum for capital adequacy purposes, Risk-Based Capital, Tier 1 Ratio	6.00%	6.00%
Excess, Risk-Based Capital, Tier 1 Ratio	6.84%	4.62%
Excess, Leverage Capital, Ratio	5.58%	4.00%
Risk-Based Capital, Total Amount
Total regulatory capital, Risk-Based Capital, Total Amount	$ 4,877,208	$ 4,277,759
Minimum for capital adequacy purposes, Risk-Based Capital, Total Amount	2,724,353	2,824,355
Excess, Risk-Based Capital, Total Amount	$ 2,152,855	$ 1,453,404
Risk-Based Capital, Total Ratio
Total regulatory capital, Risk-Based Capital, Total Ratio	14.32%	12.12%
Minimum for capital adequacy purposes, Risk-Based Capital, Total Ratio	8.00%	8.00%
Excess, Risk-Based Capital, Total Ratio	6.32%	4.12%
New York Commercial Bank
Common Equity, Tier 1 Amount
Total regulatory capital, Common Equity, Tier 1 Amount	$ 380,194	$ 370,707
Minimum for capital adequacy purposes, Common Equity, Tier 1 Amount	107,285	117,973
Excess, Common Equity, Tier 1 Amount	272,909	252,734
Total regulatory capital, Leverage Capital, Amount	$ 380,194	$ 370,707
Common Equity, Tier 1 Ratio
Total regulatory capital, Common Equity, Tier 1 Ratio	15.95%	14.14%
Minimum for capital adequacy purposes, Common Equity, Tier 1 Ratio	4.50%	4.50%
Excess, Common Equity, Tier 1 Ratio	11.45%	9.64%
Minimum for capital adequacy purposes, Leverage Capital, Amount	$ 133,801	$ 140,813
Leverage Capital, Amount
Total regulatory capital, Leverage Capital, Amount	380,194	370,707
Minimum for capital adequacy purposes, Leverage Capital, Amount	133,801	140,813
Excess, Leverage Capital, Amount	246,393	229,894
Excess, Leverage Capital, Amount	$ 246,393	$ 229,894
Leverage Capital, Ratio
Total regulatory capital, Leverage Capital, Ratio	11.37%	10.53%
Minimum for capital adequacy purposes, Leverage Capital, Ratio	4.00%	4.00%
Excess, Leverage Capital, Ratio	7.37%	6.53%
Total regulatory capital, Leverage Capital, Ratio	11.37%	10.53%
Risk-Based Capital, Tier 1 Amount
Total regulatory capital, Risk-Based Capital, Tier 1 Amount	$ 380,194	$ 370,707
Minimum for capital adequacy purposes, Risk-Based Capital, Tier 1 Amount	143,047	157,297
Excess, Risk-Based Capital, Tier 1 Amount	$ 237,147	$ 213,410
Minimum for capital adequacy purposes, Leverage Capital, Ratio	4.00%	4.00%
Risk-Based Capital, Tier 1 Ratio
Total regulatory capital, Risk-Based Capital, Tier 1 Ratio	15.95%	14.14%
Minimum for capital adequacy purposes, Risk-Based Capital, Tier 1 Ratio	6.00%	6.00%
Excess, Risk-Based Capital, Tier 1 Ratio	9.95%	8.14%
Excess, Leverage Capital, Ratio	7.37%	6.53%
Risk-Based Capital, Total Amount
Total regulatory capital, Risk-Based Capital, Total Amount	$ 404,643	$ 397,259
Minimum for capital adequacy purposes, Risk-Based Capital, Total Amount	190,729	209,729
Excess, Risk-Based Capital, Total Amount	$ 213,914	$ 187,530
Risk-Based Capital, Total Ratio
Total regulatory capital, Risk-Based Capital, Total Ratio	16.97%	15.15%
Minimum for capital adequacy purposes, Risk-Based Capital, Total Ratio	8.00%	8.00%
Excess, Risk-Based Capital, Total Ratio	8.97%	7.15%