Exchangeable Senior Notes - Interest Cost Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Contractual interest	$ 18,106	$ 19,303	$ 19,483
Amortization of discount	4,687	5,103	4,980
Total interest expense recognized	$ 22,793	$ 24,406	$ 24,463

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

+ References

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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