Accounting Policies - Changes in Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance	$ (11,019)	$ 15,533
Other comprehensive (loss)/income before reclassifications	(5,164)	41,106
Amounts reclassified from AOCI	(2,734)	(5,455)
Net other comprehensive (loss)/income	(7,898)	35,651
AOCI Ending Balance	(18,917)	51,184
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Available-for-sale Securities [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance	(10,035)	9,716
Other comprehensive (loss)/income before reclassifications	(1,769)	41,343
Amounts reclassified from AOCI	(2,376)	(3,359)
Net other comprehensive (loss)/income	(4,145)	37,984
AOCI Ending Balance	(14,180)	47,700
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Held-to-maturity Securities [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance	(476)	5,973
Other comprehensive (loss)/income before reclassifications	0	0
Amounts reclassified from AOCI	(657)	(2,173)
Net other comprehensive (loss)/income	(657)	(2,173)
AOCI Ending Balance	(1,133)	3,800
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI Beginning Balance	(508)	(156)
Other comprehensive (loss)/income before reclassifications	(3,395)	(237)
Amounts reclassified from AOCI	299	77
Net other comprehensive (loss)/income	(3,096)	(160)
AOCI Ending Balance	$ (3,604)	$ (316)