Investments Investments (Securities Lending and Repurchase Agreements) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	$ 0	$ 68,904
Total Cash collateral for loaned securities	33,169	74,976
Overnight and Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	68,904
Total Cash collateral for loaned securities	10,709	74,976
Up 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total Cash collateral for loaned securities	22,460	0
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	68,904
Total Cash collateral for loaned securities	22,460	0
U.S. Treasury securities and obligations of U.S. government authorities and agencies | Overnight and Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	68,904
Total Cash collateral for loaned securities	0	0
U.S. Treasury securities and obligations of U.S. government authorities and agencies | Up 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	0
Total Cash collateral for loaned securities	22,460	0
Foreign government bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total Cash collateral for loaned securities	6,157	6,827
Foreign government bonds | Overnight and Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total Cash collateral for loaned securities	6,157	6,827
Foreign government bonds | Up 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	4,074	62,584
U.S. corporate public securities | Overnight and Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total Cash collateral for loaned securities	4,074	62,584
U.S. corporate public securities | Up 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total Cash collateral for loaned securities	0	0
All other foreign public corporate securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total Cash collateral for loaned securities	478	5,565
All other foreign public corporate securities | Overnight and Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total Cash collateral for loaned securities	478	5,565
All other foreign public corporate securities | Up 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total Cash collateral for loaned securities	$ 0	$ 0