Federal Income Taxes - Tax Status of Dividends (Details) - $ / shares	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Ordinary income (in dollars per share)									$ 0.82	$ 0.77	$ 0.83
Qualified dividend (in dollars per share)									0.00	0.00	0.00
Capital gain (in dollars per share)									0.23	0.39	0.39
Total (in dollars per share)	$ 0.27	$ 0.26	$ 0.26	$ 0.26	$ 0.41	$ 0.25	$ 0.25	$ 0.25	$ 1.05	$ 1.16	$ 1.22
Ordinary income									78.00%	66.00%	68.00%
Qualified dividend									0.00%	0.00%	0.00%
Capital gain									22.00%	34.00%	32.00%
Total									100.00%	100.00%	100.00%