Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Cash dividends paid to parent Company by consolidated subsidiaries	$ 122.0	$ 59.0	$ 22.2
Amount available to be paid as dividends without prior regulatory approval	$ 317.7
Minimum ratio of qualifying total capital to risk-weighted assets (as a percent)	8.00%
Common Equity Tier 1 capital required for capital adequacy to risk weighted assets (as a percent)	4.50%
Tier 1 risk based capital required for capital adequacy to risk weighted assets (as a percent)	6.00%
Tier 1 leverage capital required for capital adequacy to average assets (as a percent)	4.00%
Total Capital to Risk Weighted Assets, To Be Well Capitalized by Regulatory Definition, Ratio (as a percent)	10.00%
Tier 1 Capital to Risk Weighted Assets, To Be Well Capitalized by Regulatory Definition, Ratio (as a percent)	8.00%
Common Equity Tier 1 Capital to Risk Weighted Assets, To Be Well Capitalized by Regulatory Definition, Ratio (as a percent)	6.50%
Tier 1 Leverage Ratio, To Be Well Capitalized by Regulatory Definition, Ratio (as a percent)	5.00%
Federal Reserve Bank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Cash reserve requirement	$ 557.7	$ 507.0