Accounting Policies - Changes in Accumulated Other Comprehensive Income, Net of Tax (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]
Beginning Balance	$ 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 before reclassifications	23,240	12,299
Amounts reclassified from AOCI	(2,241)	(4,513)
Net comprehensive income/(loss)	20,999	7,786
Ending Balance	72,111	41,544
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	(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 before reclassifications	18,187	12,223
Amounts reclassified from AOCI	1,417	2,578
Net comprehensive income/(loss)	16,770	9,645
Ending Balance	15,094	(4,742)
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	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 before reclassifications	0	0
Amounts reclassified from AOCI	1,035	2,267
Net comprehensive income/(loss)	(1,035)	(2,267)
Ending Balance	47,201	43,485
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	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 before reclassifications	5,053	76
Amounts reclassified from AOCI	(211)	(332)
Net comprehensive income/(loss)	5,264	408
Ending Balance	$ 9,816	$ 2,801