Acquisition - Schedule of Assets Acquired and Liabilities (Detail) - USD ($) $ in Thousands	Nov. 01, 2015	Mar. 31, 2016	Dec. 31, 2015
Identifiable assets:
Goodwill		$ 4,593,112	$ 4,593,112
Hudson City [Member]
Identifiable assets:
Cash and due from banks	$ 131,688
Interest-bearing deposits at banks	7,568,934
Investment securities	7,929,014
Loans	19,015,013
Goodwill	1,079,787
Core deposit intangible	131,665
Other assets	843,219
Total identifiable assets	36,699,320
Liabilities:
Deposits	17,879,589
Borrowings	13,211,598
Other liabilities	405,025
Total liabilities	31,496,212
Total consideration	5,203,108
Cash paid	2,064,284
Common stock awards converted	28,243
Total consideration	5,203,108
Hudson City [Member] | Common Stock [Member]
Liabilities:
Common stock issued (25,953,950 shares)	$ 3,110,581