Borrowings - Components of Borrowings (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding Balance	$ 200,000	$ 200,000
Senior notes | Series B
Debt Instrument [Line Items]
Outstanding Balance	$ 50,000	$ 50,000
Interest Rate Per Annum	5.32%	5.32%
Senior notes | Series C
Debt Instrument [Line Items]
Outstanding Balance	$ 90,000	$ 90,000
Interest Rate Per Annum	5.82%	5.82%
Senior notes | Series D
Debt Instrument [Line Items]
Outstanding Balance	$ 60,000	$ 60,000
Interest Rate Per Annum	4.29%	4.29%
Revolving credit agreement
Debt Instrument [Line Items]
Outstanding Balance	$ 0	$ 0