Financial Statement Revisions - Statements of Changes in Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Shareholders' Equity
Beginning Balances			$ 3,716.0	$ 3,716.0
Net income	$ 551.5	$ 87.7		581.1	$ 465.3
Change in foreign currency translation, net of tax	0.0	0.2		0.2	(0.4)
Ending Balances	3,337.2			3,337.2		$ 3,716.0
As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,764.1		3,737.2	3,737.2	4,368.0	4,368.0
Net income		88.6	36.4		470.4	419.8	$ 279.5	$ 290.0
Ending Balances			3,764.1			3,737.2	4,368.0
Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(29.2)		(21.2)	(21.2)	(10.4)	(10.4)
Net income		(0.9)	(6.7)		(5.1)	(10.8)	(2.8)	(7.1)
Ending Balances			(29.2)			(21.2)	(10.4)
As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,734.9		3,716.0	3,716.0	4,357.6	4,357.6
Net income		87.7	29.7		465.3	409.0	276.7	282.9
Ending Balances			3,734.9			3,716.0	4,357.6
Common shares and paid-in surplus
Increase (Decrease) in Shareholders' Equity
Beginning Balances			810.7	810.7	978.2	978.2
Repurchases and retirements of common shares				(147.9)	(188.1)
Net income				0.0	0.0
Dividends declared on common shares				0.0	0.0
Ending Balances	669.3	809.8		669.3	809.8	810.7	978.2
Common shares and paid-in surplus | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	815.1		810.7	810.7	978.2	978.2	1,034.7
Ending Balances		809.8	815.1		809.8	810.7	978.2	1,034.7
Common shares and paid-in surplus | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	0.7		0.0	0.0	0.0	0.0	0.0
Ending Balances		0.0	0.7		0.0	0.0	0.0	0.0
Common shares and paid-in surplus | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	815.8		810.7	810.7	978.2	978.2	1,034.7
Ending Balances		809.8	815.8		809.8	810.7	978.2	1,034.7
Retained earnings
Increase (Decrease) in Shareholders' Equity
Beginning Balances			2,776.6	2,776.6	3,075.0	3,075.0
Repurchases and retirements of common shares				(575.9)	(678.6)
Net income				604.7	440.7
Dividends declared on common shares				(4.6)	(5.4)
Ending Balances	2,800.8	2,831.7		2,800.8	2,831.7	2,776.6	3,075.0
Retained earnings | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	2,835.2		2,797.2	2,797.2	3,084.9	3,084.9	3,010.5	2,801.9
Repurchases and retirements of common shares			(7.2)		(678.6)	(694.8)	(217.2)	(97.4)
Net income			49.8		445.8	412.5	297.6	312.2
Dividends declared on common shares			(4.6)		(5.4)	(5.4)	(6.0)	(6.2)
Net other changes in AOCI			0.0		113.6	113.9	(34.6)	66.6
Ending Balances		2,846.7	2,835.2		2,846.7	2,797.2	3,084.9	3,010.5
Retained earnings | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(27.7)		(20.6)	(20.6)	(9.9)	(9.9)	(7.5)	(0.5)
Repurchases and retirements of common shares			0.0		0.0	0.0	0.0	0.0
Net income			(7.1)		(5.1)	(10.7)	(2.4)	(7.0)
Dividends declared on common shares			0.0		0.0	0.0	0.0	0.0
Net other changes in AOCI			0.0		0.0	0.0	0.0	0.0
Ending Balances		(15.0)	(27.7)		(15.0)	(20.6)	(9.9)	(7.5)
Retained earnings | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	2,807.5		2,776.6	2,776.6	3,075.0	3,075.0	3,003.0	2,801.4
Repurchases and retirements of common shares			(7.2)		(678.6)	(694.8)	(217.2)	(97.4)
Net income			42.7		440.7	401.8	295.2	305.2
Dividends declared on common shares			(4.6)		(5.4)	(5.4)	(6.0)	(6.2)
Net other changes in AOCI			0.0		113.6	113.9	(34.6)	66.6
Ending Balances		2,831.7	2,807.5		2,831.7	2,776.6	3,075.0	3,003.0
AOCI, after tax
Increase (Decrease) in Shareholders' Equity
Beginning Balances			(4.6)	(4.6)	(149.9)	(149.9)
Repurchases and retirements of common shares				0.0	0.0
Net income				0.0	0.0
Dividends declared on common shares				0.0	0.0
Ending Balances	(1.3)	(4.5)		(1.3)	(4.5)	(4.6)	(149.9)
AOCI, after tax | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(3.0)		(4.6)	(4.6)	(149.9)	(149.9)	(49.5)	52.1
Ending Balances		(4.3)	(3.0)		(4.3)	(4.6)	(149.9)	(49.5)
AOCI, after tax | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(1.3)		0.0	0.0	0.0	0.0	7.4	0.5
Ending Balances		(0.2)	(1.3)		(0.2)	0.0	0.0	7.4
AOCI, after tax | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(4.3)		(4.6)	(4.6)	(149.9)	(149.9)	(42.1)	52.6
Ending Balances		(4.5)	(4.3)		(4.5)	(4.6)	(149.9)	(42.1)
Total
Increase (Decrease) in Shareholders' Equity
Beginning Balances			3,582.7	3,582.7	3,903.3	3,903.3
Repurchases and retirements of common shares				(723.8)	(866.7)
Net income				604.7	440.7
Dividends declared on common shares				(4.6)	(5.4)
Ending Balances	3,468.8	3,637.0		3,468.8	3,637.0	3,582.7	3,903.3
Total | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,647.3		3,603.3	3,603.3	3,913.2	3,913.2	3,995.7
Ending Balances		3,652.2	3,647.3		3,652.2	3,603.3	3,913.2	3,995.7
Total | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(28.3)		(20.6)	(20.6)	(9.9)	(9.9)	(0.1)
Ending Balances		(15.2)	(28.3)		(15.2)	(20.6)	(9.9)	(0.1)
Total | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,619.0		3,582.7	3,582.7	3,903.3	3,903.3	3,995.6
Ending Balances		3,637.0	3,619.0		3,637.0	3,582.7	3,903.3	3,995.6
Non-controlling interest
Increase (Decrease) in Shareholders' Equity
Beginning Balances			133.3	133.3	454.3	454.3
Repurchases and retirements of common shares				(5.2)	0.0
Net income				(23.6)	24.6
Dividends declared on common shares				0.0	0.0
Ending Balances	(131.6)	148.7		(131.6)	148.7	133.3	454.3
Non-controlling interest | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	116.8		133.9	133.9	454.8	454.8	542.7	491.7
Net income			(13.4)		24.6	7.3	(18.1)	(22.2)
Other changes in NCI			(3.7)		(330.2)	(328.2)	(69.8)	73.2
Ending Balances		149.2	116.8		149.2	133.9	454.8	542.7
Non-controlling interest | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(0.9)		(0.6)	(0.6)	(0.5)	(0.5)	(0.1)	0.0
Net income			0.4		0.0	(0.1)	(0.4)	(0.1)
Other changes in NCI			(0.7)		0.0	0.0	0.0	0.0
Ending Balances		(0.5)	(0.9)		(0.5)	(0.6)	(0.5)	(0.1)
Non-controlling interest | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	115.9		133.3	133.3	454.3	454.3	542.6	491.7
Net income			(13.0)		24.6	7.2	(18.5)	(22.3)
Other changes in NCI			(4.4)		(330.2)	(328.2)	(69.8)	73.2
Ending Balances		148.7	115.9		148.7	133.3	454.3	542.6
Total Equity
Increase (Decrease) in Shareholders' Equity
Beginning Balances			3,716.0	3,716.0	4,357.6	4,357.6
Repurchases and retirements of common shares				(729.0)	(866.7)
Net income				581.1	465.3
Dividends declared on common shares				(4.6)	(5.4)
Ending Balances	3,337.2	3,785.7		3,337.2	3,785.7	3,716.0	4,357.6
Total Equity | As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,764.1		3,737.2	3,737.2	4,368.0	4,368.0	4,538.4
Ending Balances		3,801.4	3,764.1		3,801.4	3,737.2	4,368.0	4,538.4
Total Equity | Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(29.2)		(21.2)	(21.2)	(10.4)	(10.4)	(0.2)
Ending Balances		(15.7)	(29.2)		(15.7)	(21.2)	(10.4)	(0.2)
Total Equity | As revised
Increase (Decrease) in Shareholders' Equity
Beginning Balances	$ 3,734.9		3,716.0	3,716.0	4,357.6	4,357.6	4,538.2
Ending Balances		$ 3,785.7	3,734.9		3,785.7	3,716.0	4,357.6	4,538.2
Foreign Currency Gain (Loss) | Common shares and paid-in surplus
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0.0	0.0
Foreign Currency Gain (Loss) | Retained earnings
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0.0	0.0
Foreign Currency Gain (Loss) | AOCI, after tax
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0.4	32.1
Foreign Currency Gain (Loss) | AOCI, after tax | As previously reported
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax			1.6		32.0	31.4	(65.8)	(168.2)
Foreign Currency Gain (Loss) | AOCI, after tax | Adjustments
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax			(1.3)		(0.2)	0.0	(7.4)	6.9
Foreign Currency Gain (Loss) | AOCI, after tax | As revised
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax			$ 0.3		31.8	$ 31.4	$ (73.2)	$ (161.3)
Foreign Currency Gain (Loss) | Total
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0.4	32.1
Foreign Currency Gain (Loss) | Non-controlling interest
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				0.1	0.1
Foreign Currency Gain (Loss) | Total Equity
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax				$ 0.5	$ 32.2