Stockholders' Equity (Summary of Regular Quarterly Dividend) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash dividend (in usd per share)	$ 0.60	$ 0.60	$ 0.60	$ 0.60	$ 0.55	$ 2.35
Payment of dividends	$ 14,999				$ 13,858
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payment of dividends	$ 14,999	$ 15,156	$ 15,129	$ 15,127	$ 13,858	$ 59,270