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Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 3,128,869 $ 2,679,727
Gross Unrealized Holding Gain 54,079 38,537
Gross Unrealized Holding Loss (22,892) (54,622)
Fair Value 3,160,056 2,663,642
Total Percent 100.00% 100.00%
Government agency [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 342,560 350,378
Gross Unrealized Holding Gain 2 22
Gross Unrealized Holding Loss (13,363) (23,875)
Fair Value 329,199 326,525
Total Percent 10.42% 12.26%
Residential mortgage-backed securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,901,645 1,391,631
Gross Unrealized Holding Gain 22,056 13,100
Gross Unrealized Holding Loss (7,950) (24,788)
Fair Value 1,915,751 1,379,943
Total Percent 60.62% 51.81%
CMOs / REMICs - residential [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 315,298 361,573
Gross Unrealized Holding Gain 7,300 6,576
Gross Unrealized Holding Loss (643) (1,974)
Fair Value 321,955 366,175
Total Percent 10.19% 13.75%
Municipal bonds [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 564,366 571,145
Gross Unrealized Holding Gain 24,583 18,839
Gross Unrealized Holding Loss (936) (3,893)
Fair Value 588,013 586,091
Total Percent 18.61% 22.00%
Other securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 5,000 5,000
Gross Unrealized Holding Gain 138 0
Gross Unrealized Holding Loss 0 (92)
Fair Value $ 5,138 $ 4,908
Total Percent 0.16% 0.18%