CONSOLIDATED STATEMENTS OF EQUITY - USD ($) shares in Thousands, $ in Millions	Common Stock	Common Stock and Additional Paid in Capital	Retained Earnings	Accumulated Other Comprehensive (Loss) Income	Non-controlling Interests	Total
Balance at Dec. 31, 2017		$ 594.6	$ 4,144.1	$ (71.6)	$ 32.8	$ 4,699.9
Balance (in shares) at Dec. 31, 2017	72,610
Increase (Decrease) in Stockholders' Equity
Net income			169.0		2.1	171.1
Other comprehensive income (loss)				(2.9)		(2.9)
Dividends to noncontrolling interest holders					(1.8)	(1.8)
Stock-based compensation, net		1.0				1.0
Stock-based compensation, net (in shares)	269
Stock options exercised		2.8				2.8
Stock options exercised (in shares)	48
Repurchase of common shares		(49.3)				(49.3)
Repurchase of common shares (in shares)	(584)
Cash dividends - $0.55 and $0.50 per share for the quarter ended March 31, 2019 and March 31, 2018, respectively, and $0.55 and $0.50 per share for the quarter ended June 30, 2019 and June 30, 2018, respectively			(37.2)			(37.2)
Balance at Mar. 31, 2018		549.1	4,275.9	(74.5)	33.1	4,783.6
Balance (in shares) at Mar. 31, 2018	72,343
Balance at Dec. 31, 2017		594.6	4,144.1	(71.6)	32.8	4,699.9
Balance (in shares) at Dec. 31, 2017	72,610
Increase (Decrease) in Stockholders' Equity
Net income						404.2
Other comprehensive income (loss)						(14.1)
Repurchase of common shares						(50.0)
Balance at Jun. 30, 2018		561.3	4,470.6	(85.7)	34.6	4,980.8
Balance (in shares) at Jun. 30, 2018	72,350
Balance at Mar. 31, 2018		549.1	4,275.9	(74.5)	33.1	4,783.6
Balance (in shares) at Mar. 31, 2018	72,343
Increase (Decrease) in Stockholders' Equity
Net income			230.8		2.3	233.1
Other comprehensive income (loss)				(11.2)		(11.2)
Dividends to noncontrolling interest holders					(0.8)	(0.8)
Stock-based compensation, net		12.9				12.9
Stock-based compensation, net (in shares)	15
Repurchase of common shares		(0.7)				(0.7)
Repurchase of common shares (in shares)	(8)
Cash dividends - $0.55 and $0.50 per share for the quarter ended March 31, 2019 and March 31, 2018, respectively, and $0.55 and $0.50 per share for the quarter ended June 30, 2019 and June 30, 2018, respectively			(36.1)			(36.1)
Balance at Jun. 30, 2018		561.3	4,470.6	(85.7)	34.6	4,980.8
Balance (in shares) at Jun. 30, 2018	72,350
Balance at Dec. 31, 2018		136.4	4,637.9	(102.7)	7.9	$ 4,679.5
Balance (in shares) at Dec. 31, 2018	66,882					66,882
Increase (Decrease) in Stockholders' Equity
Net income			190.1		1.5	$ 191.6
Other comprehensive income (loss)				6.8		6.8
Stock-based compensation, net		(1.3)				(1.3)
Stock-based compensation, net (in shares)	333
Stock options exercised		0.8				0.8
Stock options exercised (in shares)	20
Cash dividends - $0.55 and $0.50 per share for the quarter ended March 31, 2019 and March 31, 2018, respectively, and $0.55 and $0.50 per share for the quarter ended June 30, 2019 and June 30, 2018, respectively			(38.2)			(38.2)
Balance at Mar. 31, 2019		135.9	4,789.8	(95.9)	9.4	4,839.2
Balance (in shares) at Mar. 31, 2019	67,235
Balance at Dec. 31, 2018		136.4	4,637.9	(102.7)	7.9	$ 4,679.5
Balance (in shares) at Dec. 31, 2018	66,882					66,882
Increase (Decrease) in Stockholders' Equity
Net income						$ 375.9
Other comprehensive income (loss)						8.0
Repurchase of common shares						(50.0)
Balance at Jun. 30, 2019		99.7	4,936.0	(94.7)	8.5	$ 4,949.5
Balance (in shares) at Jun. 30, 2019	66,654					66,654
Balance at Mar. 31, 2019		135.9	4,789.8	(95.9)	9.4	$ 4,839.2
Balance (in shares) at Mar. 31, 2019	67,235
Increase (Decrease) in Stockholders' Equity
Net income			183.1		1.2	184.3
Other comprehensive income (loss)				1.2		1.2
Dividends to noncontrolling interest holders					(1.7)	(1.7)
Noncontrolling interest purchased					(0.4)	(0.4)
Stock-based compensation, net		13.8				13.8
Stock-based compensation, net (in shares)	12
Repurchase of common shares		(50.0)				(50.0)
Repurchase of common shares (in shares)	(593)
Cash dividends - $0.55 and $0.50 per share for the quarter ended March 31, 2019 and March 31, 2018, respectively, and $0.55 and $0.50 per share for the quarter ended June 30, 2019 and June 30, 2018, respectively			(36.9)			(36.9)
Balance at Jun. 30, 2019		$ 99.7	$ 4,936.0	$ (94.7)	$ 8.5	$ 4,949.5
Balance (in shares) at Jun. 30, 2019	66,654					66,654