Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Total risk-based capital:
Actual Amount	$ 766,478	$ 748,431
Actual Ratio	15.90%	15.60%
Adequately Capitalized Amount	385,443	384,014
Adequately Capitalized Ratio	8.00%	8.00%
Tier 1 risk-based capital:
Actual Amount	677,787	652,929
Actual Ratio	14.10%	13.60%
Adequately Capitalized Amount	192,721	192,007
Adequately Capitalized Ratio	4.00%	4.00%
Leverage capital ratio:
Actual Ratio	9.20%	8.80%
Adequately Capitalized Amount	293,323	296,559
Adequately Capitalized Ratio	4.00%	4.00%
FIB
Total risk-based capital:
Actual Amount	707,053	697,695
Actual Ratio	14.70%	14.60%
Adequately Capitalized Amount	383,593	382,245
Adequately Capitalized Ratio	8.00%	8.00%
Well Capitalized Amount	479,491	477,806
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	634,648	622,466
Actual Ratio	13.20%	13.00%
Adequately Capitalized Amount	191,796	191,122
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	287,695	286,683
Well Capitalized Ratio	6.00%	6.00%
Leverage capital ratio:
Actual Ratio	8.70%	8.40%
Adequately Capitalized Amount	292,739	296,061
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$ 365,924	$ 370,077
Well Capitalized Ratio	5.00%	5.00%