Principles Of Preparation Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jan. 01, 2018	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from insurance settlement, investing activities	$ 4,175	$ 742
Federal statutory tax rate	21.00%	35.00%	35.00%
Decrease in accumulated other comprehensive income	$ (32,321)		$ 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	1,900
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	4,175	$ 742
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)
Decommisioning Fund Investments | US Treasury and Government | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized loss on securities	$ 3,800