Stockholders' Equity (Summary of Regular Quarterly Dividend) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash dividend (in usd per share)	$ 0.55	$ 0.55	$ 0.5	$ 0.5	$ 0.5	$ 2.05
Payment of dividends	$ 13,858				$ 12,442
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payment of dividends	$ 13,858	$ 13,711	$ 12,459	$ 12,445	$ 12,442	$ 51,057