Repurchase agreements and FHLBC Advances (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 1,853,169,104	$ 1,739,198,658	$ 113,363,873
Weighted Average Rate	1.63%	1.46%	1.74%
Weighted Average Haircut	11.50%	13.90%
Fair Value Pledged	$ 2,178,835,205	$ 2,079,752,050
Amortized Cost	2,156,470,443	2,060,654,016
Accrued Interest	7,016,242	6,734,454
Residential Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 25,544,702	$ 50,606,302
Weighted Average Rate	3.27%	2.93%
Weighted Average Funding Cost	3.79%	3.18%
Weighted Average Haircut	30.30%
Fair Value Pledged	$ 34,088,921	$ 50,686,922
Amortized Cost	32,849,686	48,426,156
Accrued Interest	45,068	53,074
Commercial Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 21,796,000	$ 42,796,000
Weighted Average Rate	2.91%	2.67%
Weighted Average Funding Cost	3.13%	3.62%
Weighted Average Haircut	33.50%	31.80%
Fair Value Pledged	$ 32,800,000	$ 62,800,000
Amortized Cost	32,798,706	62,690,578
Accrued Interest	125,314	941,247
US Treasury Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance		$ 202,362,500
Weighted Average Rate		0.42%
Weighted Average Haircut		0.57%
Fair Value Pledged		$ 203,520,859
Amortized Cost		205,763,477
Accrued Interest		693,430
Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 70,899,000
Weighted Average Rate	0.66%
Weighted Average Haircut	3.50%
Fair Value Pledged	$ 73,485,225
Amortized Cost	73,170,802
Accrued Interest	191,554
Overnight | US Treasury Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance		$ 202,362,500
Weighted Average Rate		0.42%
Weighted Average Haircut		0.57%
Fair Value Pledged		$ 203,520,859
Amortized Cost		205,763,477
Accrued Interest		693,430
30 days or less [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 961,185,000	$ 1,052,983,000
Weighted Average Rate	1.79%	1.43%
Weighted Average Haircut	14.70%	15.40%
Fair Value Pledged	$ 1,164,241,469	$ 1,268,366,695
Amortized Cost	1,152,472,020	1,256,686,536
Accrued Interest	3,851,520	4,308,583
30 days or less [Member] | Residential Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 0	$ 0
Weighted Average Rate	0.00%	0.00%
Weighted Average Funding Cost	0.00%	0.00%
Weighted Average Haircut	0.00%	0.00%
Fair Value Pledged	$ 0	$ 0
Amortized Cost	0	0
Accrued Interest	0	0
30 days or less [Member] | Commercial Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 0	$ 0
Weighted Average Rate	0.00%	0.00%
Weighted Average Funding Cost	0.00%	0.00%
Weighted Average Haircut	0.00%	0.00%
Fair Value Pledged	$ 0	$ 0
Amortized Cost	0	0
Accrued Interest	0	0
30 days or less [Member] | US Treasury Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance		$ 0
Weighted Average Rate		0.00%
Weighted Average Haircut		0.00%
Fair Value Pledged		$ 0
Amortized Cost		0
Accrued Interest		0
31-60 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 465,776,000	$ 245,124,000
Weighted Average Rate	1.23%	1.23%
Weighted Average Haircut	8.60%	11.80%
Fair Value Pledged	$ 514,624,485	$ 281,093,633
Amortized Cost	512,633,509	280,893,609
Accrued Interest	1,607,435	887,640
31-60 days [Member] | Residential Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 0	$ 0
Weighted Average Rate	0.00%	0.00%
Weighted Average Funding Cost	0.00%	0.00%
Weighted Average Haircut	0.00%	0.00%
Fair Value Pledged	$ 0	$ 0
Amortized Cost	0	0
Accrued Interest	0	0
31-60 days [Member] | Commercial Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 0	$ 0
Weighted Average Rate	0.00%	0.00%
Weighted Average Funding Cost	0.00%	0.00%
Weighted Average Haircut	0.00%	0.00%
Fair Value Pledged	$ 0	$ 0
Amortized Cost	0	0
Accrued Interest	0	0
31-60 days [Member] | US Treasury Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance		$ 0
Weighted Average Rate		0.00%
Weighted Average Haircut		0.00%
Fair Value Pledged		$ 0
Amortized Cost		0
Accrued Interest		0
61-90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 129,119,000	$ 76,739,000
Weighted Average Rate	1.69%	1.98%
Weighted Average Haircut	13.20%	21.10%
Fair Value Pledged	$ 151,989,415	$ 98,349,611
Amortized Cost	151,567,289	97,456,598
Accrued Interest	399,702	222,769
61-90 days [Member] | Residential Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 0	$ 0
Weighted Average Rate	0.00%	0.00%
Weighted Average Funding Cost	0.00%	0.00%
Weighted Average Haircut	0.00%	0.00%
Fair Value Pledged	$ 0	$ 0
Amortized Cost	0	0
Accrued Interest	0	0
61-90 days [Member] | Commercial Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 0	$ 0
Weighted Average Rate	0.00%	0.00%
Weighted Average Funding Cost	0.00%	0.00%
Weighted Average Haircut	0.00%	0.00%
Fair Value Pledged	$ 0	$ 0
Amortized Cost	0	0
Accrued Interest	0	0
61-90 days [Member] | US Treasury Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance		$ 0
Weighted Average Rate		0.00%
Weighted Average Haircut		0.00%
Fair Value Pledged		$ 0
Amortized Cost		0
Accrued Interest		0
Greater than 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 226,190,104	$ 364,352,658
Weighted Average Rate	2.00%	1.57%
Weighted Average Haircut	5.90%	9.40%
Fair Value Pledged	$ 274,494,611	$ 431,942,111
Amortized Cost	266,626,823	425,617,273
Accrued Interest	966,031	1,315,462
Greater than 90 days [Member] | Residential Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 25,544,702	$ 50,606,302
Weighted Average Rate	3.27%	2.93%
Weighted Average Funding Cost	3.79%	3.18%
Weighted Average Haircut	30.30%
Fair Value Pledged	$ 34,088,921	$ 50,686,922
Amortized Cost	32,849,686	48,426,156
Accrued Interest	45,068	53,074
Greater than 90 days [Member] | Commercial Mortgage Loans [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance	$ 21,796,000	$ 42,796,000
Weighted Average Rate	2.91%	2.67%
Weighted Average Funding Cost	3.13%	3.62%
Weighted Average Haircut	33.50%	31.80%
Fair Value Pledged	$ 32,800,000	$ 62,800,000
Amortized Cost	32,798,706	62,690,578
Accrued Interest	$ 125,314	941,247
Greater than 90 days [Member] | US Treasury Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Balance		$ 0
Weighted Average Rate		0.00%
Weighted Average Haircut		0.00%
Fair Value Pledged		$ 0
Amortized Cost		0
Accrued Interest		$ 0