Consolidated Statements Of Changes In Equity - USD ($) shares in Millions, $ in Millions	Total	Common shares	Treasury shares	Capital contribution reserve	Retained earnings	Accumulated other comprehensive income (loss)
Beginning Balance at Dec. 31, 2014	$ 4,663.8	$ 2.0	$ (1,251.9)	$ 4,250.0	$ 2,044.0	$ (380.3)
Balance (in shares) at Dec. 31, 2014		(202.4)	(19.9)
Net income (loss)	(76.4)				(76.4)
Other comprehensive income, net of tax	(264.7)					(264.7)
Tax benefit of share-based compensation	5.7			5.7
Dividends declared	(174.4)			1.5	175.9
Share repurchase (in shares)		(3.1)
Share repurchases	$ (200.0)			200.0
Treasury Stock, Shares, Retired		19.1	(19.1)
Treasury Stock, Retired, Cost Method, Amount		$ (0.2)	$ 1,210.9	(1,210.7)
Exercise of options, net of shares tendered for payment (in shares)	0.1		0.7
Exercise of options, net of shares tendered for payment	$ 31.1		$ 34.6	(3.5)
Issuance of restricted shares, net of cancellations (in shares)	0.3		0.2
Issuance of restricted shares, net of cancellations			$ 9.4	(9.4)
Shares surrendered by employees to pay taxes (in shares)	0.1		(0.1)
Shares surrendered by employees to pay taxes	$ (9.3)		$ (3.0)	(6.3)
Share-based compensation	33.0			33.0
Balance (in shares) at Dec. 31, 2015		(180.5)	0.0
Ending Balance at Dec. 31, 2015	4,008.8	$ 1.8	$ 0.0	2,860.3	1,791.7	(645.0)
Net income (loss)	522.2				522.2
Other comprehensive income, net of tax	(91.3)					(91.3)
Tax benefit of share-based compensation	5.5			5.5
Dividends declared	(245.8)			0.0	(245.8)
Exercise of options, net of shares tendered for payment (in shares)		1.0
Exercise of options, net of shares tendered for payment	31.6			31.6
Issuance of restricted shares, net of cancellations (in shares)		0.5
Issuance of restricted shares, net of cancellations				0.0
Shares surrendered by employees to pay taxes (in shares)		(0.2)
Shares surrendered by employees to pay taxes	(10.8)			(10.8)
Share-based compensation	34.2			34.2
Balance (in shares) at Dec. 31, 2016		(181.8)	0.0
Ending Balance at Dec. 31, 2016	4,254.4	$ 1.8	$ 0.0	2,920.8	2,068.1	(736.3)
Net income (loss)	666.5				666.5
Other comprehensive income, net of tax	492.9					492.9
Dividends declared	(252.9)				(252.9)
Share repurchase (in shares)		(3.0)	0.0
Share repurchases	$ (200.0)	$ 0.0	$ 0.0	(200.0)
Exercise of options, net of shares tendered for payment (in shares)	1.2	1.2
Exercise of options, net of shares tendered for payment	$ 45.6			45.6
Issuance of restricted shares, net of cancellations (in shares)		0.4
Shares surrendered by employees to pay taxes (in shares)		(0.1)
Shares surrendered by employees to pay taxes	(8.3)			(8.3)
Share-based compensation	39.6			39.6
Balance (in shares) at Dec. 31, 2017		(180.3)	0.0
Ending Balance at Dec. 31, 2017	$ 5,037.8	$ 1.8	$ 0.0	$ 2,797.7	$ 2,481.7	$ (243.4)