Dividends - Quarterly Dividend Payments (Details) - USD ($) $ / shares in Units, $ in Thousands |
3 Months Ended | 12 Months Ended | |||||
|---|---|---|---|---|---|---|---|
Dec. 31, 2017 |
Sep. 30, 2017 |
Jun. 30, 2017 |
Mar. 31, 2017 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Dividends [Abstract] | |||||||
| Cash dividends paid per common share (in dollars per share) | $ 0.19000 | $ 0.18875 | $ 0.18750 | $ 0.18625 | |||
| Total cash dividends paid | $ 13,987 | $ 13,883 | $ 13,750 | $ 13,624 | $ 55,244 | $ 53,342 | $ 51,375 |