Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Revisions to Summarized Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	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	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total equity	$ 1,036,214				$ 975,983				$ 1,036,214	$ 975,983	$ 1,635,509	$ 1,548,504
Net income	$ (1,574)	$ 11,588	$ 23,008	$ 19,938	23,347	$ 24,015	$ 21,879	$ 23,950	$ 52,960	93,191	110,519
Net transfers to/from Dover										(742,054)	(29,543)
As Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total equity					981,527					981,527	1,640,385	1,551,218
Net income		10,419	23,850	22,569	22,732	25,496	22,075	24,194		94,495	112,664
Net transfers to/from Dover										(742,690)	(29,526)
Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total equity					(5,544)					(5,544)	(4,876)	$ (2,714)
Net income		$ 1,169	$ (842)	$ (2,631)	$ 615	$ (1,481)	$ (196)	$ (244)		(1,304)	(2,145)
Net transfers to/from Dover										$ 636	$ (17)