New Accounting Standards Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019	Mar. 31, 2018	Jan. 01, 2018	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in operating loss carryforward deferred tax assets	$ 12,521	$ 24,035
Cost of reimbursable expense	899,375
Proceeds from insurance settlement, investing activities	$ 5,351	$ 9,591	$ 4,361
Federal statutory tax rate	21.00%	35.00%	35.00%
Decrease in accumulated other comprehensive income	$ 38,784	$ (11,058)
US Treasury and Government
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized loss on securities	18,601
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	8,900
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 2016-01 | US Treasury and Government | Decommisioning Fund Investments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized loss on securities	18,600
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from insurance settlement, investing activities	5,400	$ 9,600	$ 4,400
Accounting Standards Update 2016-02 | Minimum | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Leases, Future Minimum Payments Due				$ 5,000
Operating Lease, Right-of-Use Asset				5,000
Accounting Standards Update 2016-02 | Maximum | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Leases, Future Minimum Payments Due				8,000
Operating Lease, Right-of-Use Asset				$ 8,000
Potential Impact of Adopting Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings					$ 7,200
Decrease in accumulated other comprehensive income	$ 7,200