Shareholders' Equity - Reclassified out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain (Loss) on Investments									$ 6,774	$ 2,498	$ 1,163
Net impairment losses recognized in earnings									0	(1,060)	(2,364)
Income before income taxes									49,712	126,322	103,697
Income tax expense									(1,574)	(2,038)	(2,164)
Net income	$ (69,374)	$ 40,796	$ 39,887	$ 36,829	$ 30,724	$ 28,515	$ 26,511	$ 38,534	48,138	124,284	101,533
Unrealized (losses) gains on AFS securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain (Loss) on Investments									(576)	263	(3,160)
Net impairment losses recognized in earnings									0	0	(102)
Income before income taxes									(576)	263	(3,262)
Income tax expense									0	0	(16)
Net income									$ (576)	$ 263	$ (3,278)