Investment Securities - Amortized Cost, Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities
Amortized Cost	$ 2,048,583	$ 1,842,691
Gross Unrealized Gains	16,900	25,419
Gross Unrealized Losses	(50,529)	(30,490)
Fair Value	2,014,954	1,837,620
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Amortized Cost	138,638	138,676
Gross Unrealized Gains	4,093	6,581
Gross Unrealized Losses	0	0
Fair Value	142,731	145,257
U.S. government agency securities
Schedule of Held-to-maturity Securities
Amortized Cost	9,580	9,859
Gross Unrealized Gains	30	122
Gross Unrealized Losses	(173)	0
Fair Value	9,437	9,981
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Amortized Cost	354,277	244,272
Gross Unrealized Gains	5,416	7,083
Gross Unrealized Losses	(4,741)	(1,653)
Fair Value	354,952	249,702
Municipal bonds
Schedule of Held-to-maturity Securities
Amortized Cost	264,994	221,606
Gross Unrealized Gains	3,960	6,199
Gross Unrealized Losses	(1,415)	(28)
Fair Value	267,539	227,777
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Amortized Cost	619,271	465,878
Gross Unrealized Gains	9,376	13,282
Gross Unrealized Losses	(6,156)	(1,681)
Fair Value	622,491	477,479
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Amortized Cost	1,184,757	1,131,945
Gross Unrealized Gains	3,162	4,842
Gross Unrealized Losses	(36,852)	(18,743)
Fair Value	1,151,067	1,118,044
Trust preferred securities
Schedule of Held-to-maturity Securities
Amortized Cost	49,828	49,824
Gross Unrealized Gains	21	60
Gross Unrealized Losses	(7,059)	(9,796)
Fair Value	42,790	40,088
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Amortized Cost	46,509	46,509
Gross Unrealized Gains	218	532
Gross Unrealized Losses	(289)	(270)
Fair Value	$ 46,438	$ 46,771