Federal Income Taxes - Tax Status of Dividends (Details) - $ / shares	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
Income Tax Disclosure [Abstract]
Ordinary income (in dollars per share)									$ 0.77	$ 0.83	$ 0.85
Qualified dividend (in dollars per share)									0.00	0.00	0.00
Capital gain (in dollars per share)									0.39	0.39	0.38
Total (in dollars per share)	$ 0.41	$ 0.25	$ 0.25	$ 0.25	$ 0.50	$ 0.24	$ 0.24	$ 0.24	$ 1.16	$ 1.22	$ 1.23
Ordinary income									66.00%	68.00%	69.00%
Qualified dividend									0.00%	0.00%	0.00%
Capital gain									34.00%	32.00%	31.00%
Total									100.00%	100.00%	100.00%