Regulatory Capital Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations
Capital Conservation Buffer	7.13%
Total risk-based capital:
Actual Amount	$ 974,488	$ 946,156
Actual Ratio	15.13%	15.36%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 555,665
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	8.63%
Adequately Capitalized Amount	$ 676,070	$ 646,658
Adequately Capitalized Ratio	10.50%	10.50%
Well Capitalized Amount	$ 643,876	$ 615,865
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 894,273	$ 861,339
Actual Ratio	13.89%	13.99%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 426,890
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	6.63%
Adequately Capitalized Amount	$ 547,295	$ 523,485
Adequately Capitalized Ratio	8.50%	8.50%
Well Capitalized Amount	$ 515,101	$ 492,692
Well Capitalized Ratio	8.00%	8.00%
Common Equity Tier One Risked Based Capital	$ 814,273	$ 781,339
Common Equity Tier One Risk Based Capital to Risk Weighted Assets	12.65%	12.69%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 330,308	$ 277,139
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	5.13%	4.50%
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 450,713	$ 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	$ 418,519	$ 400,312
Common Equity Tier One Capital to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital	$ 894,273	$ 861,339
Tier One Leverage Capital to Average Assets	10.11%	10.12%
Leverage capital ratio:
Adequately Capitalized Amount	$ 353,838	$ 340,480
Adequately Capitalized Ratio	4.00%	4.00%
Tier One Leverage Capital Required to be Well Capitalized	$ 442,297	$ 425,600
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%
Capital Required for Capital Adequacy		$ 492,692
Capital Required for Capital Adequacy to Risk Weighted Assets		8.00%
Tier One Risk Based Capital Required for Capital Adequacy		$ 369,519
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets		6.00%
FIB
Compliance with Regulatory Capital Requirements under Banking Regulations
Capital Conservation Buffer		5.12%
Total risk-based capital:
Actual Amount	$ 841,967	$ 882,504
Actual Ratio	13.13%	14.38%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 553,459
Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	8.63%
Adequately Capitalized Amount	$ 673,386	$ 644,262
Adequately Capitalized Ratio	10.50%	10.50%
Well Capitalized Amount	$ 641,320	$ 613,582
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 765,753	$ 805,805
Actual Ratio	11.94%	13.13%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 425,195
Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	6.63%
Adequately Capitalized Amount	$ 545,122	$ 521,545
Adequately Capitalized Ratio	8.50%	8.50%
Well Capitalized Amount	$ 513,056	$ 490,866
Well Capitalized Ratio	8.00%	8.00%
Common Equity Tier One Risked Based Capital	$ 765,753	$ 805,805
Common Equity Tier One Risk Based Capital to Risk Weighted Assets	11.94%	13.13%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 328,997	$ 276,112
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule to Risk Weighted Assets	5.13%	4.50%
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 448,924	$ 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	$ 416,858	$ 398,829
Common Equity Tier One Capital to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital	$ 765,756	$ 805,805
Tier One Leverage Capital to Average Assets	8.69%	9.50%
Leverage capital ratio:
Adequately Capitalized Amount	$ 352,283	$ 339,316
Adequately Capitalized Ratio	4.00%	4.00%
Tier One Leverage Capital Required to be Well Capitalized	$ 440,353	$ 424,145
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%
Capital Required for Capital Adequacy		$ 490,866
Capital Required for Capital Adequacy to Risk Weighted Assets		8.00%
Tier One Risk Based Capital Required for Capital Adequacy		$ 368,149
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets		6.00%