Benefit Plans - Share Option Activity (Details) - Share Options [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of shares outstanding, beginning (in shares)	406,241	1,350,410
Number of shares exercised (in shares)	(271,451)	(944,169)
Number of shares outstanding, ending (in shares)	134,790	406,241
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Price per share outstanding, beginning (usd per share)	$ 6.78	$ 7.05
Price per share exercised (usd per share)	6.48	7.17
Price per share outstanding, ending (usd per share)	$ 7.39	$ 6.78