Borrowings Borrowings - Schedule of Debt Maturities (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Total	$ 1,644,400
Less than 1 Year	0
1-3 Years	1,494,400
3-5 Years	100,000
After 5 Years	50,000
Senior Notes - 2007 [Member]
Debt Instrument [Line Items]
Total	250,000
Less than 1 Year	0
1-3 Years	100,000
3-5 Years	100,000
After 5 Years	50,000
Senior Notes - 2013 [Member]
Debt Instrument [Line Items]
Total	300,000
Less than 1 Year	0
1-3 Years	300,000
3-5 Years	0
After 5 Years	0
Senior Notes - 2016 [Member]
Debt Instrument [Line Items]
Total	594,400
Less than 1 Year	0
1-3 Years	594,400
3-5 Years	0
After 5 Years	0
Senior notes - 2017 [Member]
Debt Instrument [Line Items]
Total	500,000
Less than 1 Year	0
1-3 Years	500,000
3-5 Years	0
After 5 Years	$ 0