Consolidated Statements of Capital - USD ($) shares in Millions, $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Beginning Balance		$ 2,574.7		$ 2,333.7	$ 2,574.7	$ 2,333.7	$ 2,291.8
Net income	$ 50.5	58.0	$ 10.8	50.1	173.5	130.4	197.7
Other comprehensive income (loss), after tax					3.5	(2.5)	17.8
Common stock issued					5.0	300.8	5.9
Cash dividends declared					(211.7)	(199.2)	(191.2)
Stock compensation expense					13.1	12.7	13.5
Restricted stock—dividends					1.3	1.2	1.1
Tax short fall—stock compensation					(0.4)	(2.4)	(2.9)
Ending Balance	2,559.0		2,574.7		2,559.0	2,574.7	2,333.7
Common Stock [Member]
Beginning Balance		$ 234.9		$ 217.3	$ 234.9	$ 217.3	$ 216.6
Beginning Balance, shares		234.9		217.3	234.9	217.3	216.6
Common stock issued					$ 0.4	$ 17.6	$ 0.7
Common stock issued, shares					0.4	17.6	0.7
Ending Balance	$ 235.3		$ 234.9		$ 235.3	$ 234.9	$ 217.3
Ending Balance, shares	235.3		234.9		235.3	234.9	217.3
Additional Paid-in Capital [Member]
Beginning Balance		$ 1,875.9		$ 1,581.3	$ 1,875.9	$ 1,581.3	$ 1,564.5
Common stock issued					4.6	283.2	5.2
Stock compensation expense					13.1	12.7	13.5
Restricted stock—dividends					1.3	1.1	1.0
Tax short fall—stock compensation					(0.4)	(2.4)	(2.9)
Ending Balance	$ 1,894.5		$ 1,875.9		1,894.5	1,875.9	1,581.3
Retained Earnings [Member]
Beginning Balance		479.6		548.3	479.6	548.3	541.7
Net income					173.5	130.4	197.7
Cash dividends declared					(211.7)	(199.2)	(191.2)
Restricted stock—dividends						0.1	0.1
Ending Balance	441.4		479.6		441.4	479.6	548.3
Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance		$ (15.7)		$ (13.2)	(15.7)	(13.2)	(31.0)
Other comprehensive income (loss), after tax					3.5	(2.5)	17.8
Ending Balance	$ (12.2)		$ (15.7)		$ (12.2)	$ (15.7)	$ (13.2)