Securities (Detail 4) $ in Thousands	Sep. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($) security item
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months	$ 990,556	$ 2,027,804
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months	4,090	6,728
Securities available-for-sale, Fair Value, 12 months or greater	1,266,874	1,472,436
Securities available-for-sale, Estimated Unrealized Loss, 12 months or greater	18,824	37,586
Securities available-for-sale, Fair Value, Total	2,257,430	3,500,240
Securities available-for-sale, Estimated Unrealized Loss, Total	22,914	44,314
Securities held-to-maturity:
Securities held-to-maturity, Fair Value, Less than 12 months	291,472	283,478
Securities held-to-maturity, Estimated Unrealized Loss, Less than 12 months	2,349	2,568
Securities held-to-maturity, Fair Value, 12 months or greater	250,473	824,838
Securities held-to-maturity, Estimated Unrealized Loss, 12 months or greater	2,751	12,475
Securities held-to-maturity, Fair Value, Total	541,945	1,108,316
Securities held-to-maturity, Estimated Unrealized Loss, Total	5,100	15,043
U.S. Treasury
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months	17,001	15,814
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months	1	10
Securities available-for-sale, Fair Value, Total	17,001	15,814
Securities available-for-sale, Estimated Unrealized Loss, Total	$ 1	$ 10
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | security	1	2
Federal agency - Debt
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months	$ 17,556	$ 1,008,234
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months	14	2,622
Securities available-for-sale, Fair Value, 12 months or greater		135,868
Securities available-for-sale, Estimated Unrealized Loss, 12 months or greater		658
Securities available-for-sale, Fair Value, Total	17,556	1,144,102
Securities available-for-sale, Estimated Unrealized Loss, Total	14	3,280
Securities held-to-maturity:
Securities held-to-maturity, Fair Value, 12 months or greater	5,739	26,316
Securities held-to-maturity, Estimated Unrealized Loss, 12 months or greater	27	255
Securities held-to-maturity, Fair Value, Total	5,739	26,316
Securities held-to-maturity, Estimated Unrealized Loss, Total	$ 27	$ 255
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | security	3	37
Federal agency - MBS
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months		$ 35
Securities available-for-sale, Fair Value, 12 months or greater	$ 53,639	57,970
Securities available-for-sale, Estimated Unrealized Loss, 12 months or greater	660	1,014
Securities available-for-sale, Fair Value, Total	53,639	58,005
Securities available-for-sale, Estimated Unrealized Loss, Total	660	1,014
Securities held-to-maturity:
Securities held-to-maturity, Fair Value, Less than 12 months	21,649	4,800
Securities held-to-maturity, Estimated Unrealized Loss, Less than 12 months	25	21
Securities held-to-maturity, Fair Value, 12 months or greater	26,077	114,856
Securities held-to-maturity, Estimated Unrealized Loss, 12 months or greater	202	2,254
Securities held-to-maturity, Fair Value, Total	47,726	119,656
Securities held-to-maturity, Estimated Unrealized Loss, Total	$ 227	$ 2,275
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | security	13	23
CMOs - Federal agency
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months	$ 690,496	$ 871,026
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months	3,947	3,417
Securities available-for-sale, Fair Value, 12 months or greater	1,197,026	1,261,695
Securities available-for-sale, Estimated Unrealized Loss, 12 months or greater	17,792	35,452
Securities available-for-sale, Fair Value, Total	1,887,522	2,132,721
Securities available-for-sale, Estimated Unrealized Loss, Total	21,739	38,869
Securities held-to-maturity:
Securities held-to-maturity, Fair Value, Less than 12 months	136,388	202,014
Securities held-to-maturity, Estimated Unrealized Loss, Less than 12 months	596	2,247
Securities held-to-maturity, Fair Value, 12 months or greater	184,818	588,019
Securities held-to-maturity, Estimated Unrealized Loss, 12 months or greater	1,461	8,045
Securities held-to-maturity, Fair Value, Total	321,206	790,033
Securities held-to-maturity, Estimated Unrealized Loss, Total	$ 2,057	$ 10,292
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | security	113	141
CMOs - Non-agency
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months		$ 1,949
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months		15
Securities available-for-sale, Fair Value, 12 months or greater	$ 12,739	12,720
Securities available-for-sale, Estimated Unrealized Loss, 12 months or greater	343	396
Securities available-for-sale, Fair Value, Total	12,739	14,669
Securities available-for-sale, Estimated Unrealized Loss, Total	$ 343	$ 411
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | security	3	3
State and municipal
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months	$ 65,670	$ 130,208
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months	99	314
Securities available-for-sale, Fair Value, 12 months or greater	3,470	4,183
Securities available-for-sale, Estimated Unrealized Loss, 12 months or greater	29	66
Securities available-for-sale, Fair Value, Total	69,140	134,391
Securities available-for-sale, Estimated Unrealized Loss, Total	128	380
Securities held-to-maturity:
Securities held-to-maturity, Fair Value, Less than 12 months	118,335	14,851
Securities held-to-maturity, Estimated Unrealized Loss, Less than 12 months	1,727	76
Securities held-to-maturity, Fair Value, 12 months or greater	33,839	95,647
Securities held-to-maturity, Estimated Unrealized Loss, 12 months or greater	1,061	1,921
Securities held-to-maturity, Fair Value, Total	152,174	110,498
Securities held-to-maturity, Estimated Unrealized Loss, Total	$ 2,788	$ 1,997
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | security	196	225
Other debt securities
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months	$ 199,833
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months	29
Securities available-for-sale, Fair Value, Total	199,833
Securities available-for-sale, Estimated Unrealized Loss, Total	29
Securities held-to-maturity:
Securities held-to-maturity, Fair Value, Less than 12 months	15,100	$ 61,813
Securities held-to-maturity, Estimated Unrealized Loss, Less than 12 months	1	224
Securities held-to-maturity, Fair Value, Total	15,100	61,813
Securities held-to-maturity, Estimated Unrealized Loss, Total	$ 1	$ 224
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | security	3	5
Total debt securities
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months		$ 2,027,266
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months		6,378
Securities available-for-sale, Fair Value, 12 months or greater		1,472,436
Securities available-for-sale, Estimated Unrealized Loss, 12 months or greater		37,586
Securities available-for-sale, Fair Value, Total		3,499,702
Securities available-for-sale, Estimated Unrealized Loss, Total		$ 43,964
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | security	332	436
Equity securities and mutual funds
Securities available-for-sale:
Securities available-for-sale, Fair Value, Less than 12 months		$ 538
Securities available-for-sale, Estimated Unrealized Loss, Less than 12 months		350
Securities available-for-sale, Fair Value, Total		538
Securities available-for-sale, Estimated Unrealized Loss, Total		$ 350
Other information
Number of available-for-sale and held-to-maturity securities in the unrealized loss position | item		1