Accounting Policies - Changes in Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 33,758	$ (11,019)	$ 15,533
Other comprehensive loss before reclassifications	24,077	55,062	(7,181)
Amounts reclassified from AOCI	15,803	10,285	19,371
Net comprehensive loss	8,274	44,777	(26,552)
Reclass of Stranded Tax Effects from AOCI to Retained Earnings	0
Ending Balance	51,085	33,758	(11,019)
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	(14,387)	(10,035)	9,716
Other comprehensive loss before reclassifications	23,925	5,481	(6,026)
Amounts reclassified from AOCI	10,917	9,833	13,725
Net comprehensive loss	13,008	(4,352)	(19,751)
Reclass of Stranded Tax Effects from AOCI to Retained Earnings	297
Ending Balance	(1,676)	(14,387)	(10,035)
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	45,752	(476)	5,973
Other comprehensive loss before reclassifications	0	47,993	0
Amounts reclassified from AOCI	6,064	1,765	6,449
Net comprehensive loss	(6,064)	46,228	(6,449)
Reclass of Stranded Tax Effects from AOCI to Retained Earnings	(8,548)
Ending Balance	48,236	45,752	(476)
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	2,393	(508)	(156)
Other comprehensive loss before reclassifications	152	1,588	(1,155)
Amounts reclassified from AOCI	(1,178)	(1,313)	(803)
Net comprehensive loss	1,330	2,901	(352)
Reclass of Stranded Tax Effects from AOCI to Retained Earnings	(802)
Ending Balance	4,525	2,393	(508)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net comprehensive loss	8,274	$ 44,777	$ (26,552)
Reclass of Stranded Tax Effects from AOCI to Retained Earnings	$ 9,053