Accumulated Other Comprehensive Income Reclassification out of AOCI to RE (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (21,313)		$ (8,658)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	588	$ 36
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	11
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	14	(180)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	14	(169)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(12,895)	2,083
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	574	216
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	0	(12)
Other comprehensive income/ (loss), net of tax	(12,321)	2,311
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(21,313)	(10,237)	(8,658)	$ (12,548)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(12,307)	2,142
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	14	(169)
Other comprehensive income/ (loss), net of tax	(12,321)	2,311
Cumulative Effect of New Accounting Principle in Period of Adoption	334
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(21,374)	(9,111)	(8,140)	(11,194)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Reclassification Adjustments, Net of Tax	(12,895)	2,094
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	11
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(12,895)	2,083
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Accounting Standards Update 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	339
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	911	(389)	332	(605)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	588	36
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	14	(180)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	574	216
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Accounting Standards Update 2017-12 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(5)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(850)	(737)	$ (850)	$ (749)
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment, after Tax	0	(12)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	0	$ (12)
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 0