Consolidated Statements of Changes in Equity - USD ($) $ in Millions	Total	Common stock	Retained earnings	Derivative Hedging Instrument	AOCI	Non-controlling interests
Beginning balance (in shares) at Dec. 31, 2017		142,094,275
Beginning balance at Dec. 31, 2017	$ 3,860.4	$ 2,734.8	$ 1,173.3		$ 0.0	$ (47.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	62.9		60.5			2.4
Issuance of stock for compensation and reinvested dividends (in shares)		138,828
Issuance of stock for compensation and reinvested dividends	(3.7)	$ (3.7)
Dividends declared on common stock	(57.7)		(57.7)
Stock compensation expense	2.5	$ 2.5
Net current period other comprehensive income	0.0
Ending balance (in shares) at Mar. 31, 2018		142,233,103
Ending balance at Mar. 31, 2018	3,864.4	$ 2,733.6	1,176.1		0.0	(45.3)
Beginning balance (in shares) at Dec. 31, 2018		255,326,252
Beginning balance at Dec. 31, 2018	9,990.7	$ 8,685.2	1,346.0		(3.0)	(37.5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	103.4		99.5			3.9
Issuance of stock for compensation and reinvested dividends (in shares)		60,594
Issuance of stock for compensation and reinvested dividends	(1.6)	$ (1.6)
Dividends declared on common stock	(119.8)		(119.8)
Stock compensation expense	5.4	$ 5.4
Repurchase of common stock (in shares)		(10,548,060)
Repurchase of common stock under repurchase plan	(578.3)	$ (578.3)
Consolidation of noncontrolling interests	3.8					3.8
Distributions to shareholders of noncontrolling interests	(1.4)					(1.4)
Net current period other comprehensive income	(10.2)			$ (10.2)
Other	(0.3)	$ (0.3)
Ending balance (in shares) at Mar. 31, 2019		244,838,786
Ending balance at Mar. 31, 2019	$ 9,391.7	$ 8,110.4	$ 1,325.7		$ (13.2)	$ (31.2)