Disclosures About Fair Value of Financial Instruments (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Disclosures About Fair Value of Financial Instruments
Significant unrealized appreciation or depreciation in financial instruments	$ 0	$ 0
Financial assets:
Securities available-for-sale	916,423,000	912,371,000
Securities held-to-maturity	92,989,000	106,292,000
Accrued interest receivable	19,753,000	18,623,000
Financial liabilities:
Borrowed funds	943,842,000	888,109,000
Junior subordinated debentures	103,094,000	103,094,000
Assets transferred between Level 1 and Level 2	0
Liabilities transferred between Level 1 and Level 2	0
Level 1
Financial assets:
Cash and cash equivalents	297,101,000	240,706,000
Securities available-for-sale	2,924,000	3,157,000
Accrued interest receivable	19,753,000	18,623,000
Total financial assets	319,778,000	262,486,000
Financial liabilities:
Savings and checking accounts	4,253,299,000	4,154,228,000
Borrowed funds	143,460,000	162,714,000
Accrued interest payable	1,336,000	936,000
Total financial liabilities	4,398,095,000	4,317,878,000
Level 2
Financial assets:
Securities available-for-sale	903,193,000	898,617,000
Securities held-to-maturity	92,989,000	106,292,000
Total financial assets	996,182,000	1,004,909,000
Financial liabilities:
Cash flow hedges - swaps	6,205,000	6,273,000
Total financial liabilities	6,205,000	6,273,000
Level 3
Financial assets:
Securities available-for-sale	10,306,000	10,597,000
Loans receivable, net	6,323,248,000	6,240,079,000
Total financial assets	6,333,554,000	6,250,676,000
Financial liabilities:
Time deposits	1,446,470,000	1,498,539,000
Borrowed funds	832,407,000	756,898,000
Junior subordinated debentures	109,373,000	109,435,000
Total financial liabilities	2,388,250,000	2,364,872,000
Carrying amount
Financial assets:
Cash and cash equivalents	297,101,000	240,706,000
Securities available-for-sale	916,423,000	912,371,000
Securities held-to-maturity	90,825,000	103,695,000
Loans receivable, net	5,997,566,000	5,922,373,000
Accrued interest receivable	19,753,000	18,623,000
FHLB Stock	36,292,000	33,293,000
Total financial assets	7,357,960,000	7,231,061,000
Financial liabilities:
Savings and checking accounts	4,253,299,000	4,154,228,000
Time deposits	1,428,768,000	1,478,314,000
Borrowed funds	943,842,000	888,109,000
Junior subordinated debentures	103,094,000	103,094,000
Cash flow hedges - swaps	6,205,000	6,273,000
Accrued interest payable	1,336,000	936,000
Total financial liabilities	6,736,544,000	6,630,954,000
Estimated fair value
Financial assets:
Cash and cash equivalents	297,101,000	240,706,000
Securities available-for-sale	916,423,000	912,371,000
Securities held-to-maturity	92,989,000	106,292,000
Loans receivable, net	6,323,248,000	6,240,079,000
Accrued interest receivable	19,753,000	18,623,000
FHLB Stock	36,292,000	33,293,000
Total financial assets	7,685,806,000	7,551,364,000
Financial liabilities:
Savings and checking accounts	4,253,299,000	4,154,228,000
Time deposits	1,446,470,000	1,498,539,000
Borrowed funds	975,867,000	919,612,000
Junior subordinated debentures	109,373,000	109,435,000
Cash flow hedges - swaps	6,205,000	6,273,000
Accrued interest payable	1,336,000	936,000
Total financial liabilities	$ 6,792,550,000	$ 6,689,023,000