Shareholders' Equity - Distributions (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended								12 Months Ended
	Feb. 20, 2020	Jan. 16, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Share Distributions
Annual Per Share Distribution (in dollars per share)			$ 0.54	$ 0.54	$ 0.54	$ 0.53	$ 0.53	$ 0.53	$ 0.53	$ 0.52	$ 2.15	$ 2.11	$ 2.07
Total Distribution											$ 353,620	$ 346,832	$ 340,084
Ordinary Income											44.50%	100.00%	94.69%
Capital Gain											37.93%	0.00%	0.00%
Return of Capital											17.57%	0.00%	4.39%
Qualified Dividend											0.50%	0.00%	0.92%
Subsequent event
Common Share Distributions
Annual Per Share Distribution (in dollars per share)	$ 0.54
Total Distribution		$ 88,864
Cash distributions paid or payable (in dollars per share)		$ 0.54