Investments (Repurchase Agreement and Securities Lending) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	$ 13,576	$ 33,169
30 days or greater
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	0
Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	13,576	10,709
Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	22,460
US Treasury securities and obligations of U.S. government authorities and agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	4,887	22,460
US Treasury securities and obligations of U.S. government authorities and agencies | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	4,887	0
US Treasury securities and obligations of U.S. government authorities and agencies | Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	22,460
Foreign government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	3,003	6,157
Foreign government bonds | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	3,003	6,157
Foreign government bonds | Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	0
U.S. corporate public securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	5,237	4,074
U.S. corporate public securities | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	5,237	4,074
U.S. corporate public securities | Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	0
US private corporate securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	0
US private corporate securities | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	0
US private corporate securities | Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	0
Foreign corporate public securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	449	478
Foreign corporate public securities | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	449	478
Foreign corporate public securities | Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	$ 0	$ 0