Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Total risk-based capital:
Actual Amount	$ 930,048	$ 946,156
Actual Ratio	15.00%	15.40%
Well Capitalized Amount	$ 618,456	$ 615,865
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 848,709	$ 861,339
Actual Ratio	13.70%	14.00%
Well Capitalized Amount	$ 494,765	$ 492,692
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 768,709	$ 781,339
Actual Ratio	12.40%	12.70%
Leverage capital ratio:
Actual Amount	$ 848,709	$ 861,339
Actual Ratio	10.10%	10.10%
Adequately Capitalized Amount	$ 337,137	$ 340,480
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 421,421	$ 425,600
Well Capitalized Ratio	5.00%	5.00%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 533,419	$ 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	$ 649,379	$ 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	$ 409,727	$ 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	$ 525,688	$ 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	$ 316,959	$ 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	$ 432,919	$ 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	$ 401,997	$ 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	$ 876,265	$ 882,504
Actual Ratio	14.20%	14.40%
Well Capitalized Amount	$ 616,224	$ 613,582
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 799,201	$ 805,805
Actual Ratio	13.00%	13.10%
Well Capitalized Amount	$ 492,979	$ 490,866
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 799,201	$ 805,805
Actual Ratio	13.00%	13.10%
Leverage capital ratio:
Actual Amount	$ 799,201	$ 805,805
Actual Ratio	9.50%	9.50%
Adequately Capitalized Amount	$ 336,258	$ 339,316
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 420,322	$ 424,145
Well Capitalized Ratio	5.00%	5.00%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 531,493	$ 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	$ 647,035	$ 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	$ 408,248	$ 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	$ 523,790	$ 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	$ 315,815	$ 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	$ 431,357	$ 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	$ 400,546	$ 398,829
Common Equity Tier One Capital to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%