Common Stock Capitalization (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended				9 Months Ended		16 Months Ended
	Jul. 26, 2019	Jul. 23, 2019	Sep. 30, 2019		Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Jul. 22, 2019	Jun. 30, 2019	Feb. 28, 2019	Dec. 31, 2018	Jun. 30, 2018	Jun. 13, 2018	Feb. 06, 2018	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Common stock, shares authorized (in shares)			100,000,000				100,000,000		100,000,000				100,000,000
Common shares			$ 0.001				$ 0.001		$ 0.001			$ 0.001
Common stock, shares outstanding			33,774,386			30,155,055	33,774,386	30,155,055	33,774,386		29,745,776		29,796,601	30,149,880			31,335,938
Dividends declared (in usd per share)			$ 0.42			$ 0.41	$ 1.39	$ 1.23
Dividends declared			$ 14,500				$ 44,100
Dividends paid						$ (12,700)	$ (39,226)	$ (38,152)
Common shares issued and outstanding upon conversion of all convertible securities (in shares)			34,518,699			30,871,043	34,518,699	30,871,043	34,518,699
Number of shares authorized to be repurchased (in shares)															1,550,000	1,550,000
Shares repurchased (in shares)			0			0	50,825	1,186,058	411,915
Average price per share (USD per share)							$ 15.39	$ 14.83	$ 15.34
Total cost					$ (782)	$ 0	$ (800)	$ (17,593)	$ (6,300)
Stock Issued During Period, Shares, New Issues	525,000	3,500,000	4,025,000	[1]			4,025,000
Proceeds from issuance of shares, net	$ 9,100									$ 60,700

[1] Net of underwriters' discount and offering costs.