Investments (Securities Pledged as Collateral Related to Repurchase Reverse Repurchase Program) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	$ 121,551	$ 73,625
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	428,344	499,891
Securities Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	576,786	624,032
Secured Borrowings, Gross, Difference, Amount	26,891	50,516
Obligation to Return Securities Received as Collateral	549,895	573,516
Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0	0
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	1,131	1,246
Obligation to Return Securities Received as Collateral	1,131	1,246
Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0	0
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Obligation to Return Securities Received as Collateral	0	0
Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0	4,017
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	312	3,220
Obligation to Return Securities Received as Collateral	312	7,237
Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	121,551	69,608
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	426,901	495,425
Obligation to Return Securities Received as Collateral	548,452	565,033
Corporate Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	121,551	73,625
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	184,646	170,199
Corporate Debt Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0	0
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Corporate Debt Securities [Member] | Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0	0
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Corporate Debt Securities [Member] | Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0	4,017
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	312	3,220
Corporate Debt Securities [Member] | Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	121,551	69,608
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	184,334	166,979
Residential Mortgage Backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	92,546
Residential Mortgage Backed Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Residential Mortgage Backed Securities [Member] | Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Residential Mortgage Backed Securities [Member] | Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Residential Mortgage Backed Securities [Member] | Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	92,546
US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	220,765	216,000
US Government Agencies Debt Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
US Government Agencies Debt Securities [Member] | Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
US Government Agencies Debt Securities [Member] | Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
US Government Agencies Debt Securities [Member] | Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	220,765	216,000
Foreign Government Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	21,802	19,900
Foreign Government Debt Securities [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Foreign Government Debt Securities [Member] | Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Foreign Government Debt Securities [Member] | Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Foreign Government Debt Securities [Member] | Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	21,802	19,900
Other Debt Obligations [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	1,131	1,246
Other Debt Obligations [Member] | Maturity Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	1,131	1,246
Other Debt Obligations [Member] | Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Other Debt Obligations [Member] | Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	0	0
Other Debt Obligations [Member] | Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Financial Assets Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement	$ 0	$ 0