Common Equity - Restricted Shares (Details) - Restricted stock - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Stock Activity [Roll Forward]
Outstanding, shares, beginning of period	220,046	229,018
Granted, shares		146,941
Released, shares		(141,224)
Forfeited, shares		(14,689)
Outstanding, shares, end of period		220,046	229,018
Restricted Stock Weighted-Average Grant Date Fair Value
Outstanding, weighted-average grant date fair value, beginning of period	$ 51.30	$ 46.78
Granted, weighted-average grant date fair value		53.69
Released, weighted-average grant date fair value		46.14
Forfeited, weighted-average grant date fair value		54.39
Outstanding, weighted-average grant date fair value, end of period		$ 51.30	$ 46.78
Restricted Stock - Additional Disclosures
Intrinsic value of released restricted shares		$ 7.7	$ 3.7	$ 2.7
Actual tax benefit realized for the tax deductions		3.1	$ 1.3	$ 1.0
Compensation cost not yet recognized		$ 5.1
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 11 months
Subsequent event
Restricted Stock Activity [Roll Forward]
Granted, shares	82,622
Restricted Stock Weighted-Average Grant Date Fair Value
Granted, weighted-average grant date fair value	$ 58.10