Accounting Policies - Changes in Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 73,410	$ 42,428	$ 51,085	$ 33,758
Cumulative Effect of New Accounting Principle in Period of Adoption					$ 27	$ 0
Accumulated Other Comprehensive Income, Net of Tax, Adjusted Balance					51,112	33,758
Other comprehensive (loss)/income before reclassifications	(7,285)	2,201	19,943	19,189
Amounts reclassified from AOCI	(2,124)	(3,834)	(7,054)	(12,152)
Net comprehensive (loss)/income	(9,409)	(1,633)	12,889	7,037
Ending Balance	64,001	40,795	64,001	40,795
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	15,882	(1,276)	(1,676)	(14,387)
Cumulative Effect of New Accounting Principle in Period of Adoption					0	0
Accumulated Other Comprehensive Income, Net of Tax, Adjusted Balance					(1,676)	(14,387)
Other comprehensive (loss)/income before reclassifications	(9,947)	2,298	10,450	20,711
Amounts reclassified from AOCI	(755)	(2,875)	(3,594)	(8,177)
Net comprehensive (loss)/income	(10,702)	(577)	6,856	12,534
Ending Balance	5,180	(1,853)	5,180	(1,853)
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Held-to-maturity Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	45,979	42,104	48,236	45,752
Cumulative Effect of New Accounting Principle in Period of Adoption					0	0
Accumulated Other Comprehensive Income, Net of Tax, Adjusted Balance					48,236	45,752
Other comprehensive (loss)/income before reclassifications	0	0	0	0
Amounts reclassified from AOCI	(1,220)	(1,221)	(3,477)	(4,869)
Net comprehensive (loss)/income	(1,220)	(1,221)	(3,477)	(4,869)
Ending Balance	44,759	40,883	44,759	40,883
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	11,549	1,600	4,525	2,393
Cumulative Effect of New Accounting Principle in Period of Adoption					27	0
Accumulated Other Comprehensive Income, Net of Tax, Adjusted Balance					$ 4,552	$ 2,393
Other comprehensive (loss)/income before reclassifications	2,662	(97)	9,493	(1,522)
Amounts reclassified from AOCI	(149)	262	17	894
Net comprehensive (loss)/income	2,513	165	9,510	(628)
Ending Balance	14,062	1,765	14,062	1,765
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gain (Loss) on Derivative Instruments [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, Current Period, Tax	0	(1,514)	0	(4,365)
Amounts reclassified from AOCI	0	2,812	0	8,105
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	0	(4,326)	0	(12,470)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Gain (Loss) on Investments [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, Current Period, Tax	0	(31)	0	(31)
Amounts reclassified from AOCI	0	58	0	58
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	$ 0	$ (89)	$ 0	$ (89)