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Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 2,741,367 $ 2,679,727
Gross Unrealized Holding Gain 45,422 38,537
Gross Unrealized Holding Loss (36,726) (54,622)
Fair Value 2,750,063 2,663,642
Total Percent 100.00% 100.00%
Government agency [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 335,412 350,378
Gross Unrealized Holding Gain 3 22
Gross Unrealized Holding Loss (18,586) (23,875)
Fair Value 316,829 326,525
Total Percent 11.52% 12.26%
Residential mortgage-backed securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 1,511,226 1,391,631
Gross Unrealized Holding Gain 16,365 13,100
Gross Unrealized Holding Loss (15,415) (24,788)
Fair Value 1,512,176 1,379,943
Total Percent 54.99% 51.81%
CMO's / REMIC's - residential [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 344,470 361,573
Gross Unrealized Holding Gain 7,466 6,576
Gross Unrealized Holding Loss (563) (1,974)
Fair Value 351,373 366,175
Total Percent 12.78% 13.75%
Municipal bonds [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 545,259 571,145
Gross Unrealized Holding Gain 21,588 18,839
Gross Unrealized Holding Loss (2,162) (3,893)
Fair Value 564,685 586,091
Total Percent 20.53% 22.00%
Other securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 5,000 5,000
Gross Unrealized Holding Gain      
Gross Unrealized Holding Loss    (92)
Fair Value $ 5,000 $ 4,908
Total Percent 0.18% 0.18%