Shareholders' Equity - Cash Dividends Declared Per Common Share (Details) - $ / shares	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2012	Dec. 31, 2017	Dec. 31, 2016
Disclosure Shareholders Equity [Abstract]
Quarterly dividend declared and paid per common share	$ 0.58	$ 0.55	$ 2.32	$ 2.2