Commercial Paper (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Carrying value	$ 99,049	$ 93,371
Commercial paper
Short-term Debt [Line Items]
Face value	99,049	93,371
Carrying value	$ 99,049	$ 93,371
Minimum | Commercial paper
Short-term Debt [Line Items]
Maturity range	10 days	8 days
Effective interest rate (as a percent)	0.70%	0.50%
Maximum | Commercial paper
Short-term Debt [Line Items]
Maturity range	30 days	28 days
Effective interest rate (as a percent)	0.80%	0.60%