Related Party Transactions (Debt Agreements) (Details) - Affiliated Entity - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 971,899	$ 971,899
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 18,734	18,734
Interest Rate	2.60%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 25,000	25,000
Interest Rate	2.60%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 46,835	46,835
Interest Rate	2.80%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 18,734	18,734
Interest Rate	2.80%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 37,468	37,468
Interest Rate	3.64%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 93,671	93,671
Interest Rate	3.64%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 103,039	103,039
Interest Rate	3.64%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 93,671	93,671
Interest Rate	3.47%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 93,671	93,671
Interest Rate	4.39%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 28,102	28,102
Interest Rate	4.39%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 37,468	37,468
Interest Rate	3.95%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 93,671	93,671
Interest Rate	3.95%
Prudential Insurance Loan Issued 4/20/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 46,835	46,835
Interest Rate	3.95%
Prudential Insurance Loan Issued 6/28/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 30,000	30,000
Interest Rate	2.08%
Prudential Insurance Loan Issued 6/28/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 50,000	50,000
Interest Rate	3.87%
Prudential Insurance Loan Issued 6/28/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 25,000	25,000
Interest Rate	3.49%
Prudential Insurance Loan Issued 6/28/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 26,000	26,000
Interest Rate	2.59%
Prudential Insurance Loan Issued 6/28/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 25,000	25,000
Interest Rate	2.08%
Prudential Insurance Loan Issued 6/28/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 20,000	20,000
Interest Rate	2.08%
Prudential Insurance Loan Issued 6/28/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 25,000	25,000
Interest Rate	3.49%
Prudential Retirement Insurance & Annuity Issued 6/28/2016
Related Party Transaction [Line Items]
Short-term and Long-term debt	$ 34,000	$ 34,000
Interest Rate	3.09%