Long-Term Debt Shelf Arrangements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Long-term Debt	$ (327,200)	$ (205,200)
Debt Instrument, Unused Borrowing Capacity, Amount	250,000
Prudential [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount	150,000
Long-term Debt	(70,000)
Debt Instrument, Unused Borrowing Capacity, Amount	50,000
MetLife [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Unused Borrowing Capacity, Amount	150,000
New York Life [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount	100,000
Debt Instrument, Unused Borrowing Capacity, Amount	50,000
Aggregate Shelf Agreements [Member]
Line of Credit Facility [Line Items]
Long-term Debt	(170,000)
Aggregated Unfunded Commitments [Member]
Line of Credit Facility [Line Items]
Long-term Debt	(100,000)
Uncollateralized Senior Note to be Issued in 2019 [Member]
Line of Credit Facility [Line Items]
Long-term Debt	$ (100,000)
Debt Instrument, Interest Rate, Stated Percentage	3.98%
New York Life [Member]
Line of Credit Facility [Line Items]
Long-term Debt	$ (100,000)
Uncollateralized Senior Note Due April Two Thousand Thirty Two [Member]
Line of Credit Facility [Line Items]
Long-term Debt	(70,000)	$ (70,000)
Maximum [Member] | Prudential [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount	220,000
Maximum [Member] | MetLife [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount	150,000
Maximum [Member] | New York Life [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount	150,000
Maximum [Member] | Aggregate Shelf Agreements [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount	$ 520,000