Subsequent Events (Details)				3 Months Ended				12 Months Ended
	Jun. 30, 2018 USD ($)	Feb. 15, 2018 USD ($) $ / shares	Feb. 01, 2017	Dec. 31, 2017 $ / shares	Sep. 30, 2017 $ / shares	Jun. 30, 2017 $ / shares	Mar. 31, 2017 $ / shares	Dec. 31, 2017 $ / shares	Dec. 31, 2016 $ / shares	Dec. 31, 2015 $ / shares	Jun. 01, 2017 USD ($)
Subsequent Event [Line Items]
Cash dividends declared per common share (in $ per share) | $ / shares				$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.80	$ 1.8	$ 1.8
Cash dividends declared per preferred share (in $ per share) | $ / shares				$ 1.8125	$ 1.8125	$ 1.8125	$ 1.8125	$ 7.25	$ 0	$ 0
Subsequent Event
Subsequent Event [Line Items]
Cash dividends declared per common share (in $ per share) | $ / shares		$ 0.45
Cash dividends declared per preferred share (in $ per share) | $ / shares		$ 1.8125
Term Loan | Additional Term Loan 2018 | Subsequent Event
Subsequent Event [Line Items]
Maximum borrowing capacity		$ 105,000,000.0
Covenant terms, leverage ratio		2.5
Covenant terms, amount drawn down		$ 30,000,000
Term Loan | Credit Facility 2017
Subsequent Event [Line Items]
Maximum borrowing capacity											$ 260,000,000.0
Revolving Credit Facility | Credit Facility 2017
Subsequent Event [Line Items]
Maximum borrowing capacity											$ 100,000,000.0
Convertible preferred stock
Subsequent Event [Line Items]
Preferred stock dividend rate			7.25%
Forecast
Subsequent Event [Line Items]
Purchase price, cash	$ 129,500,000
London Interbank Offered Rate (LIBOR) | Credit Facility 2017
Subsequent Event [Line Items]
Basis spread on variable interest rate								3.75%
London Interbank Offered Rate (LIBOR) | Credit Facility 2017 | Subsequent Event
Subsequent Event [Line Items]
Reduction in rate		1.25%
Basis spread on variable interest rate		2.50%
Minimum | London Interbank Offered Rate (LIBOR) | Credit Facility 2017
Subsequent Event [Line Items]
Basis spread on variable interest rate								3.50%
Minimum | London Interbank Offered Rate (LIBOR) | Credit Facility 2017 | Subsequent Event
Subsequent Event [Line Items]
Basis spread on variable interest rate		2.25%
Maximum | London Interbank Offered Rate (LIBOR) | Credit Facility 2017
Subsequent Event [Line Items]
Basis spread on variable interest rate								3.75%
Maximum | London Interbank Offered Rate (LIBOR) | Credit Facility 2017 | Subsequent Event
Subsequent Event [Line Items]
Basis spread on variable interest rate		2.50%