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Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 2,609,959 $ 2,374,816
Gross Unrealized Holding Gain 48,493 76,539
Gross Unrealized Holding Loss (41,145) (1,968)
Fair Value 2,617,307 2,449,387
Total Percent 100.00% 100.00%
Government agency [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 353,580 357,960
Gross Unrealized Holding Gain 36 1,588
Gross Unrealized Holding Loss (20,603) (248)
Fair Value 333,013 359,300
Total Percent 12.72% 14.67%
Residential mortgage-backed securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 1,286,448 862,196
Gross Unrealized Holding Gain 19,249 25,529
Gross Unrealized Holding Loss (16,073) (127)
Fair Value 1,289,624 887,598
Total Percent 49.28% 36.24%
CMO/REMICs [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 383,268 565,968
Gross Unrealized Holding Gain 7,002 7,402
Gross Unrealized Holding Loss (1,062) (1,410)
Fair Value 389,208 571,960
Total Percent 14.87% 23.35%
Municipal bonds [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 581,663 583,692
Gross Unrealized Holding Gain 22,206 41,920
Gross Unrealized Holding Loss (3,145) (183)
Fair Value 600,724 625,429
Total Percent 22.95% 25.53%
Other Securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 5,000 5,000
Gross Unrealized Holding Gain   100
Gross Unrealized Holding Loss (262)  
Fair Value $ 4,738 $ 5,100
Total Percent 0.18% 0.21%