Fair Value - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) Region	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Region	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Nonfinancial Assets and Liabilities [Abstract]
Outstanding balances in impaired loans	$ 28.6		$ 28.6		$ 26.5
Key market regions | Region	5		5
Foreclosed assets measured at fair value			$ 22.0	$ 26.8
Allowance for possible loan losses, Charge-offs for foreclosed assets upon initial recognition			19.8	7.4
Foreclosed assets re-measured after initial recognition			26.5	39.4
Write-downs of allowance for foreclosed assets after initial recognition			3.7	2.4
Noninterest gain (loss) Mortgage banking, net for changes in fair value of LHFS	$ (1.1)	$ 3.6	2.6	1.7
Interest earned on LHFS included in Interest and fees on LHFS and LHFI	1.5	$ 1.4	3.7	$ 3.7
GNMA optional repurchase loans	$ 37.3		$ 37.3		$ 36.0
Maximum [Member]
Nonfinancial Assets and Liabilities [Abstract]
Estimated fair value of financial instruments with immediate and shorter-term maturities			90 days