Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended								12 Months Ended
	Feb. 12, 2018	Feb. 06, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Dividends declared per common share (usd per share)			$ 0.05	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.40	$ 1.8	$ 1.80
Share repurchase program, September 2017 | Common stock
Subsequent Event [Line Items]
Authorized repurchase amount				$ 285.0
Remaining authorization under share repurchase plan			$ 219.2								$ 219.2
Subsequent event
Subsequent Event [Line Items]
Dividends declared per common share (usd per share)		$ 0.05
Dividend payable date		Mar. 21, 2018
Dividend record date		Mar. 07, 2018
Subsequent event | Share repurchase program, September 2017 | Common stock
Subsequent Event [Line Items]
Remaining authorization under share repurchase plan	$ 212.7
Increase (decrease) in stock repurchase program, authorization if the tender off is fully subscribed	$ 102.7
Number of business days before repurchase program will be implemented through other means	10 days
Subsequent event | Stock repurchase program, September 2017, modified Dutch auction | Common stock
Subsequent Event [Line Items]
Authorized repurchase amount	$ 110.0
Subsequent event | Stock repurchase program, September 2017, modified Dutch auction | Common stock | Minimum
Subsequent Event [Line Items]
Amount shareholders can offer to sell their shares, modified Dutch auction (in dollars per share)	$ 18.50
Subsequent event | Stock repurchase program, September 2017, modified Dutch auction | Common stock | Maximum
Subsequent Event [Line Items]
Incremental threshold in which shareholders can sell their shares, modified Dutch auction (in dollars per share)	$ 20.50