Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Total risk-based capital:
Actual Amount	$ 957,008	$ 974,488
Actual Ratio	14.94%	15.13%
Well Capitalized Amount	$ 640,626	$ 643,876
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 880,777	$ 894,273
Actual Ratio	13.75%	13.89%
Well Capitalized Amount	$ 512,501	$ 515,101
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 800,777	$ 814,273
Actual Ratio	12.50%	12.65%
Leverage capital ratio:
Actual Amount	$ 880,777	$ 894,273
Actual Ratio	10.09%	10.11%
Adequately Capitalized Amount	$ 349,244	$ 353,838
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 436,555	$ 442,297
Well Capitalized Ratio	5.00%	5.00%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 592,579	$ 555,665
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	9.25%	8.63%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 672,658	$ 676,070
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 464,454	$ 426,890
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	7.25%	6.63%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 544,532	$ 547,295
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%	8.50%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 368,360	$ 330,308
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	5.75%	5.13%
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 448,438	$ 450,713
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%	7.00%
Common Equity Tier One Capital to be Well Capitalized	$ 416,407	$ 418,519
Common Equity Tier One Capital to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
FIB
Total risk-based capital:
Actual Amount	$ 861,953	$ 841,967
Actual Ratio	13.52%	13.13%
Well Capitalized Amount	$ 637,759	$ 641,320
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 785,722	$ 765,753
Actual Ratio	12.32%	11.94%
Well Capitalized Amount	$ 510,207	$ 513,056
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 785,722	$ 765,753
Actual Ratio	12.32%	11.94%
Leverage capital ratio:
Actual Amount	$ 785,722	$ 765,753
Actual Ratio	9.02%	8.69%
Adequately Capitalized Amount	$ 348,525	$ 352,283
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 435,656	$ 440,353
Well Capitalized Ratio	5.00%	5.00%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 589,927	$ 553,459
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	9.25%	8.63%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 669,646	$ 673,386
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 462,375	$ 425,195
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	7.25%	6.63%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 542,095	$ 545,122
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%	8.50%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 366,711	$ 328,997
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	5.75%	5.13%
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 446,431	$ 448,924
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%	7.00%
Common Equity Tier One Capital to be Well Capitalized	$ 414,543	$ 416,858
Common Equity Tier One Capital to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%