Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ 5,269,455
Accumulated other comprehensive loss at end of period	5,419,145
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		$ (2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	246	264
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(22,780)	(23,836)
Net current period other comprehensive loss	246	262
Accumulated other comprehensive loss at end of period	(22,534)	(23,574)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(219)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1	0
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(220)	0
Net current period other comprehensive loss	1	(219)
Accumulated other comprehensive loss at end of period	(219)	(219)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(221)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	247	264
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,000)	(23,836)
Net current period other comprehensive loss	247	43
Accumulated other comprehensive loss at end of period	$ (22,753)	$ (23,793)