Debt Cash Pool (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
QRS Cash Pool [Member]
Debt Instrument [Line Items]
Cash pool agreement, gross cash position	$ 271,400	$ 300,800
Cash pool agreement, outstanding borrowings	(269,400)	(298,800)
Cash pool agreement, net cash position	2,000	2,000
TRS Cash Pool [Member]
Debt Instrument [Line Items]
Cash pool agreement, gross cash position	281,100	281,500
Cash pool agreement, outstanding borrowings	(277,900)	(279,300)
Cash pool agreement, net cash position	$ 3,200	$ 2,200