Earnings Per Share - Participating Securities (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic EPS:
Average common shares outstanding	183,917	182,971	183,633	182,656
Basic EPS	$ 0.67	$ 0.46	$ 1.87	$ 1.34
Diluted EPS:
Average common and common equivalent shares outstanding(d)	185,808	183,782	185,473	183,485
Diluted EPS	$ 0.66	$ 0.46	$ 1.85	$ 1.33
Participating Securities
Basic EPS:
Net income allocated to nonvested awards	$ 231	$ 391	$ 871	$ 1,240
Average common shares outstanding	354	860	475	945
Basic EPS	$ 0.65	$ 0.45	$ 1.83	$ 1.31
Diluted EPS:
Net income allocated to nonvested awards	$ 229	$ 389	$ 867	$ 1,236
Average common and common equivalent shares outstanding(d)	354	860	475	945
Diluted EPS	$ 0.65	$ 0.45	$ 1.83	$ 1.31