Notes and Bonds Payable - Details of Mortgage Notes (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) mortgage_note_payable	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Number of outstanding mortgage notes | mortgage_note_payable	18
Mortgage Notes [Member]
Debt Instrument [Line Items]
Investment in Collateral	$ 400.9
Mortgage notes payable carrying amount	$ 155.4	$ 115.6
Number of outstanding mortgage notes | mortgage_note_payable	19
Mortgage Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Contractual interest rates	6.90%
Mortgage Notes [Member] | Minimum [Member]
Debt Instrument [Line Items]
Contractual interest rates	3.30%
Mortgage Notes [Member] | Mortgage Notes 5.53% [Member] | Life Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 7.0
Effective Interest rate	5.53%
Maturity Date	2018-01
Collateral	MOB
Principal and Interest Payments	Monthly/15-yr amort.
Principal and interest amortization period	15 years
Investment in Collateral	$ 0.0
Mortgage notes payable carrying amount	0.0	0.7
Unemcumbered gross investment	14.3
Mortgage Notes [Member] | Mortgage Notes 5.55% [Member] | Commercial Bank [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 1.8
Effective Interest rate	5.55%
Maturity Date	2030-10
Collateral	OTH
Principal and Interest Payments	Monthly/27-yr amort.
Principal and interest amortization period	27 years
Investment in Collateral	$ 0.0
Mortgage notes payable carrying amount	0.0	1.3
Unemcumbered gross investment	8.0
Mortgage Notes [Member] | Mortgage Notes 5.54% [Member] | Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 7.3
Effective Interest rate	5.54%
Maturity Date	2018-12
Collateral	MOB
Principal and Interest Payments	Monthly/25-yr amort.
Principal and interest amortization period	25 years
Investment in Collateral	$ 14.3
Mortgage notes payable carrying amount	6.0	6.2
Unamortized premium	0.6
Mortgage Notes [Member] | Mortgage Notes 5.07% [Member] | Commercial Bank [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 9.5
Effective Interest rate	5.07%
Maturity Date	2019-03
Collateral	MOB
Principal and Interest Payments	Monthly/5-yr amort.
Principal and interest amortization period	5 years
Investment in Collateral	$ 13.9
Mortgage notes payable carrying amount	9.3	9.5
Unamortized premium	0.2
Mortgage Notes [Member] | Mortgage Notes 4.55% [Member] | Commercial Bank [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 9.4
Effective Interest rate	4.55%
Maturity Date	2019-07
Collateral	MOB
Principal and Interest Payments	Monthly/8-yr amort
Principal and interest amortization period	8 years
Investment in Collateral	$ 27.8
Mortgage notes payable carrying amount	9.2	9.3
Unamortized premium	0.3
Mortgage Notes [Member] | Mortgage Notes 7.65% [Member] | Commercial Bank [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 15.2
Effective Interest rate	7.65%
Maturity Date	2020-07
Collateral	MOB
Principal and Interest Payments	-21
Principal and interest amortization period	11 years
Investment in Collateral	$ 20.2
Mortgage notes payable carrying amount	12.7	12.7
Unaccreted discount	$ 2.4
Period for monthly installments of interest	24 months
Mortgage Notes [Member] | Mortgage Notes 4.00% [Member] | Life Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 7.9
Effective Interest rate	4.00%
Maturity Date	2020-08
Collateral	MOB
Principal and Interest Payments	Monthly/15-yr amort.
Principal and interest amortization period	15 years
Investment in Collateral	$ 20.7
Mortgage notes payable carrying amount	2.0	2.7
Unamortized premium	0.3
Mortgage Notes [Member] | Mortgage Notes 5.25% [Member] | Life Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 7.3
Effective Interest rate	5.25%
Maturity Date	2020-08
Collateral	MOB
Principal and Interest Payments	Monthly/27-yr amort.
Principal and interest amortization period	27 years
Investment in Collateral	$ 17.9
Mortgage notes payable carrying amount	6.5	6.7
Unamortized premium	0.4
Mortgage Notes [Member] | Mortgage Notes 4.27% [Member] | Life Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 5.6
Effective Interest rate	4.27%
Maturity Date	2021-01
Collateral	MOB
Principal and Interest Payments	Monthly/10-yr amort.
Principal and interest amortization period	10 years
Investment in Collateral	$ 15.7
Mortgage notes payable carrying amount	4.8	0.0
Unamortized premium	0.2
Mortgage Notes [Member] | Mortgage Notes 6.43% [Member] | Commercial Bank [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 12.9
Effective Interest rate	6.43%
Maturity Date	2021-02
Collateral	MOB
Principal and Interest Payments	Monthly/12-yr amort.
Principal and interest amortization period	12 years
Investment in Collateral	$ 54.9
Mortgage notes payable carrying amount	10.5	10.7
Unaccreted discount	1.0
Mortgage Notes [Member] | Mortgage Notes 3.85% [Member] | Life Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 11.0
Effective Interest rate	3.85%
Maturity Date	2022-11
Collateral	MOB
Principal and Interest Payments	Monthly/7-yr amort.
Principal and interest amortization period	7 years
Investment in Collateral	$ 22.0
Mortgage notes payable carrying amount	10.4	0.0
Unaccreted discount	0.1
Mortgage Notes [Member] | Mortgage Notes 3.85% [Member] | Life Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 12.3
Effective Interest rate	3.85%
Maturity Date	2023-08
Collateral	MOB
Principal and Interest Payments	Monthly/7-yr amort.
Principal and interest amortization period	7 years
Investment in Collateral	$ 24.6
Mortgage notes payable carrying amount	11.5	0.0
Unaccreted discount	0.2
Mortgage Notes [Member] | Mortgage Notes 4.27% [Member] | Financial Services [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 12.4
Effective Interest rate	4.27%
Maturity Date	2023-10
Collateral	MOB
Principal and Interest Payments	Monthly/10-yr amort.
Principal and interest amortization period	10 years
Investment in Collateral	$ 24.7
Mortgage notes payable carrying amount	12.2	0.0
Unamortized premium	0.4
Mortgage Notes [Member] | Mortgage Notes 4.13% [Member] | Life Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 13.3
Effective Interest rate	4.13%
Maturity Date	2024-01
Collateral	MOB
Principal and Interest Payments	Monthly/10-yr amort.
Principal and interest amortization period	10 years
Investment in Collateral	$ 21.1
Mortgage notes payable carrying amount	13.3	13.6
Unamortized premium	0.8
Mortgage Notes [Member] | Mortgage Notes 3.94% [Member] | Life Insurance Co. [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 6.8
Effective Interest rate	3.94%
Maturity Date	2024-02
Collateral	MOB
Principal and Interest Payments	Monthly/7-yr amort.
Principal and interest amortization period	7 years
Investment in Collateral	$ 14.5
Mortgage notes payable carrying amount	6.7	0.0
Unamortized premium	0.2
Mortgage Notes [Member] | Mortgage Notes 4.32% [Member] | Financial Services [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 9.7
Effective Interest rate	4.32%
Maturity Date	2024-09
Collateral	MOB
Principal and Interest Payments	Monthly/10-yr amort.
Principal and interest amortization period	10 years
Investment in Collateral	$ 16.4
Mortgage notes payable carrying amount	8.8	9.1
Unamortized premium	0.1
Mortgage Notes [Member] | Mortgage Notes 3.71% [Member] | Commercial Bank [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 11.5
Effective Interest rate	3.71%
Maturity Date	2026-01
Collateral	MOB
Principal and Interest Payments	Monthly/10-yr amort.
Principal and interest amortization period	10 years
Investment in Collateral	$ 37.9
Mortgage notes payable carrying amount	10.5	11.0
Mortgage Notes [Member] | Mortgage Notes 5.25% 20 Year [Member] | Commercial Bank [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 15.0
Effective Interest rate	5.25%
Maturity Date	2027-04
Collateral	MOB
Principal and Interest Payments	Monthly/20-yr amort.
Principal and interest amortization period	20 years
Investment in Collateral	$ 33.4
Mortgage notes payable carrying amount	9.6	10.4
Unamortized premium	0.7
Mortgage Notes [Member] | Mortgage Notes 4.79% [Member] | Municipal Government [Member]
Debt Instrument [Line Items]
Mortgage notes	$ 11.0
Effective Interest rate	4.79%
Collateral	MOB
Principal and Interest Payments	Semi-Annual (20)
Investment in Collateral	$ 20.9
Mortgage notes payable carrying amount	11.4	$ 11.7
Unamortized premium	$ 1.0
Number of outstanding mortgage notes | mortgage_note_payable	3