CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Retained Earnings	Accumulated other comprehensive income (loss)
Balance at beginning of period (in shares) at Dec. 31, 2014		82.6
Balance at beginning of period at Dec. 31, 2014	$ 1,606.6	$ 715.7	$ 892.2	$ (1.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	197.3		197.3
Other comprehensive income (loss)	0.1			0.1
Common stock:
Issuance: option exercises & dividend reinvestment plan (in shares)		0.2
Issuance: option exercises & dividend reinvestment plan	6.2	$ 6.2
Dividends	(127.3)		(127.3)
Other	0.9	$ 0.9
Balance at end of period (in shares) at Dec. 31, 2015		82.8
Balance at end of period at Dec. 31, 2015	1,683.8	$ 722.8	962.2	(1.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	211.6		211.6
Other comprehensive income (loss)	(0.1)			(0.1)
Common stock:
Issuance: option exercises & dividend reinvestment plan (in shares)		0.1
Issuance: option exercises & dividend reinvestment plan	6.3	$ 6.3
Dividends	(134.2)		(134.2)
Other	$ 0.7	$ 0.7
Balance at end of period (in shares) at Dec. 31, 2016	82.9	82.9
Balance at end of period at Dec. 31, 2016	$ 1,768.1	$ 729.8	1,039.6	(1.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	216.0		216.0
Other comprehensive income (loss)	0.0
Common stock:
Issuance: option exercises & dividend reinvestment plan (in shares)		0.1
Issuance: option exercises & dividend reinvestment plan	6.3	$ 6.3
Dividends	(141.9)		(141.9)
Other	$ 0.8	$ 0.8
Balance at end of period (in shares) at Dec. 31, 2017	83.0	83.0
Balance at end of period at Dec. 31, 2017	$ 1,849.3	$ 736.9	$ 1,113.7	$ (1.3)