Investments (Credit Quality Indicators) (Details) - Commercial and agricultural mortgage loans - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	$ 1,234,182	$ 436,075
LTV, 0%-59.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	688,582	313,280
LTV, 60% to 69.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	410,545	95,977
LTV, 70%-79.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	124,263	26,818
LTV, Greater than or Equal to 80%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	10,792	0
DSCR, Greater than or Equal to 1.2X
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	1,192,274	424,593
DSCR, Greater than or Equal to 1.2X | LTV, 0%-59.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	667,051	303,215
DSCR, Greater than or Equal to 1.2X | LTV, 60% to 69.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	406,728	95,977
DSCR, Greater than or Equal to 1.2X | LTV, 70%-79.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	108,770	25,401
DSCR, Greater than or Equal to 1.2X | LTV, Greater than or Equal to 80%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	9,725	0
DSCR, 1.0X to 1.2X
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	32,414	10,490
DSCR, 1.0X to 1.2X | LTV, 0%-59.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	16,921	9,073
DSCR, 1.0X to 1.2X | LTV, 60% to 69.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	0	0
DSCR, 1.0X to 1.2X | LTV, 70%-79.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	15,493	1,417
DSCR, 1.0X to 1.2X | LTV, Greater than or Equal to 80%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	0	0
DSCR, Less Than 1.0X
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	9,494	992
DSCR, Less Than 1.0X | LTV, 0%-59.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	4,610	992
DSCR, Less Than 1.0X | LTV, 60% to 69.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	3,817	0
DSCR, Less Than 1.0X | LTV, 70%-79.99%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	0	0
DSCR, Less Than 1.0X | LTV, Greater than or Equal to 80%
Credit Quality Indicators [Line Items]
Financing Receivable, Gross	$ 1,067	$ 0