Commercial Paper (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Carrying value	$ 99,886	$ 99,049
Commercial paper
Short-term Debt [Line Items]
Face value	99,886	99,049
Carrying value	$ 99,886	$ 99,049
Minimum | Commercial paper
Short-term Debt [Line Items]
Maturity range	19 days	10 days
Effective interest rate (as a percent)	1.40%	0.70%
Maximum | Commercial paper
Short-term Debt [Line Items]
Maturity range	67 days	30 days
Effective interest rate (as a percent)	1.70%	0.80%