Accounting Policies - Changes in Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 72,111	$ 41,544	$ 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 income/(loss) before reclassifications	3,987	4,691	27,227	16,987
Amounts reclassified from AOCI	(2,688)	(3,807)	(4,929)	8,317
Net comprehensive income/(loss)	1,299	884	22,298	8,670
Ending Balance	73,410	42,428	73,410	42,428
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	15,094	(4,742)	(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 income/(loss) before reclassifications	2,209	6,191	20,396	18,413
Amounts reclassified from AOCI	1,421	2,725	2,838	5,302
Net comprehensive income/(loss)	788	3,466	17,558	13,111
Ending Balance	15,882	(1,276)	15,882	(1,276)
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	47,201	43,485	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 income/(loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI	1,222	1,381	2,257	3,648
Net comprehensive income/(loss)	(1,222)	(1,381)	(2,257)	(3,648)
Ending Balance	45,979	42,104	45,979	42,104
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	9,816	2,801	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 income/(loss) before reclassifications	1,778	(1,500)	6,831	(1,426)
Amounts reclassified from AOCI	45	(299)	(166)	(633)
Net comprehensive income/(loss)	1,733	(1,201)	6,997	(793)
Ending Balance	$ 11,549	$ 1,600	$ 11,549	$ 1,600