Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,329,892	$ 2,374,816
Gross Unrealized Holding Gain	66,709	76,539
Gross Unrealized Holding Loss	(5,928)	(1,968)
Fair Value	2,390,673	2,449,387
Total Percent	100.00%	100.00%
Government agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	391,336	357,960
Gross Unrealized Holding Gain	845	1,588
Gross Unrealized Holding Loss	(1,543)	(248)
Fair Value	390,638	359,300
Total Percent	16.34%	14.67%
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	874,913	862,196
Gross Unrealized Holding Gain	21,139	25,529
Gross Unrealized Holding Loss	(3,415)	(127)
Fair Value	892,637	887,598
Total Percent	37.34%	36.24%
CMO's / REMIC's - residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	470,694	565,968
Gross Unrealized Holding Gain	7,439	7,402
Gross Unrealized Holding Loss	(556)	(1,410)
Fair Value	477,577	571,960
Total Percent	19.98%	23.35%
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	587,949	583,692
Gross Unrealized Holding Gain	37,264	41,920
Gross Unrealized Holding Loss	(414)	(183)
Fair Value	624,799	625,429
Total Percent	26.13%	25.53%
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,000	5,000
Gross Unrealized Holding Gain	22	100
Fair Value	$ 5,022	$ 5,100
Total Percent	0.21%	0.21%