Shareholders' Equity - Narrative (Details) - USD ($)					1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 23, 2018	Oct. 09, 2018	Mar. 14, 2018	Mar. 15, 2017	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Stock repurchase program, period in force			12 months
Common stock, dividend declared (in dollars per share)						$ 2.50	$ 0	$ 2.50	$ 0
Subsequent event
Class of Stock [Line Items]
Common stock, dividend declared (in dollars per share)		$ 2.5
Common stock, cash dividends paid	$ 304,700,000
Common Shares
Class of Stock [Line Items]
Stock repurchase program, authorized amount (up to)				$ 150,000,000
Stock repurchase program, period in force				12 months
Stock repurchase program, amount not utilized					$ 81,000,000
Stock repurchase program, additional authorized amount (up to)			$ 150,000,000
Stock repurchased and retired during period (in shares)								2,970,209
Stock repurchased and retired during period, price per share (in dollars per share)								$ 29.67
Stock repurchased and retired during period, value			$ 19,100,000	$ 69,000,000				$ 88,100,000
Stock repurchase program, remaining authorized repurchase amount						$ 130,900,000		$ 130,900,000
Number of shares surrendered to satisfy tax withholding obligations (in shares)								57,348	6,694