Principles Of Preparation Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Jan. 01, 2018	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of reimbursable expense	$ 299,369		$ 710,510		$ 905,063
Proceeds from insurance settlement, investing activities			$ 5,351	$ 8,069
Federal statutory tax rate	21.00%	35.00%	21.00%	35.00%		35.00%
Decrease in accumulated other comprehensive income	$ (34,633)		$ (34,633)		(34,633)	$ 11,058
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in operating loss carryforward deferred tax assets								$ 200
Increase in retained earnings								$ 200
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of reimbursable expense			5,900
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings							$ 41,000
Decrease in accumulated other comprehensive income							$ (41,000)
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from insurance settlement, investing activities			5,400	$ 8,100
Potential Impact of Adopting Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in accumulated other comprehensive income	(7,200)		(7,200)		$ (7,200)
US Treasury and Government
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized gain on securities	$ 6,528		3,730
Decommisioning Fund Investments | US Treasury and Government | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized gain on securities			$ 3,700