Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Total risk-based capital:
Actual Amount	$ 1,100,588	$ 974,488
Actual Ratio	12.76%	15.13%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 797,584	$ 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	$ 905,365	$ 676,070
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Well Capitalized Amount	$ 862,253	$ 643,876
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,026,016	$ 894,273
Actual Ratio	11.90%	13.89%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 625,133	$ 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	$ 732,915	$ 547,295
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%	8.50%
Well Capitalized Amount	$ 689,802	$ 515,101
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 949,821	$ 814,273
Actual Ratio	11.02%	12.65%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 495,795	$ 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%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 603,577	$ 450,713
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%	7.00%
Well Capitalized Amount	$ 560,464	$ 418,519
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,026,016	$ 894,273
Actual Ratio	8.71%	10.11%
Adequately Capitalized Amount	$ 471,221	$ 353,838
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 589,026	$ 442,297
Well Capitalized Ratio	5.00%	5.00%
FIB
Total risk-based capital:
Actual Amount	$ 1,097,287	$ 841,967
Actual Ratio	12.78%	13.13%
Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 793,974	$ 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	$ 901,268	$ 673,386
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	10.50%	10.50%
Well Capitalized Amount	$ 858,350	$ 641,320
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	$ 1,022,715	$ 765,753
Actual Ratio	11.91%	11.94%
Tier One Risk Based Capital under Basel III Phase-in Schedule	$ 622,304	$ 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	$ 729,598	$ 545,122
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	8.50%	8.50%
Well Capitalized Amount	$ 686,680	$ 513,056
Well Capitalized Ratio	8.00%	8.00%
Common equity tier one risk-based capital:
Actual Amount	$ 1,022,715	$ 765,753
Actual Ratio	11.91%	11.94%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Phase-in Schedule	$ 493,551	$ 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%
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in	$ 600,845	$ 448,924
Common Equity Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets	7.00%	7.00%
Well Capitalized Amount	$ 557,928	$ 416,858
Well Capitalized Ratio	6.50%	6.50%
Leverage capital ratio:
Actual Amount	$ 1,022,715	$ 765,753
Actual Ratio	8.72%	8.69%
Adequately Capitalized Amount	$ 469,107	$ 352,283
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 586,384	$ 440,353
Well Capitalized Ratio	5.00%	5.00%