Recently Issued Accounting Standards Recently Issued Accounting Standards (Details) - USD ($)		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle	[1]		$ 4,113,000
Restricted cash				$ 0	$ 75,000,000
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle		$ 4,100,000

[1] Related to adoption of ASU No. 2016-01. See Notes K and S for additional information.