Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Total risk-based capital:
Actual Amount	$ 962,182	$ 946,156
Actual Ratio	14.90%	15.40%
Well Capitalized Amount	$ 647,138	$ 615,865
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 877,285	$ 861,339
Actual Ratio	13.60%	14.00%
Well Capitalized Amount	$ 517,711	$ 492,692
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 797,285	$ 781,339
Actual Ratio	12.30%	12.70%
Leverage capital ratio:
Actual Amount	$ 877,285	$ 861,339
Actual Ratio	10.20%	10.10%
Adequately Capitalized Amount	$ 343,228	$ 340,480
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 429,035	$ 425,600
Well Capitalized Ratio	5.00%	5.00%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 558,157	$ 492,692
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	8.60%	8.00%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 679,495	$ 646,658
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	$ 428,729	$ 369,519
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	6.60%	6.00%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 550,068	$ 523,485
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	$ 331,658	$ 277,139
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	5.10%	4.50%
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 452,997	$ 431,105
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	$ 420,640	$ 400,312
Common Equity Tier One Capital to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
FIB
Total risk-based capital:
Actual Amount	$ 898,288	$ 882,504
Actual Ratio	13.90%	14.40%
Well Capitalized Amount	$ 644,816	$ 613,582
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 817,678	$ 805,805
Actual Ratio	12.70%	13.10%
Well Capitalized Amount	$ 515,853	$ 490,866
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 817,678	$ 805,805
Actual Ratio	12.70%	13.10%
Leverage capital ratio:
Actual Amount	$ 817,678	$ 805,805
Actual Ratio	9.50%	9.50%
Adequately Capitalized Amount	$ 342,629	$ 339,316
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 428,286	$ 424,145
Well Capitalized Ratio	5.00%	5.00%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 556,154	$ 490,866
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	8.60%	8.00%
Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 677,057	$ 644,262
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	$ 427,191	$ 368,149
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	6.60%	6.00%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 548,094	$ 521,545
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	$ 330,468	$ 276,112
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	5.10%	4.50%
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 451,371	$ 429,508
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	$ 419,131	$ 398,829
Common Equity Tier One Capital to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%