Debt Obligations - Summary (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Obligations
Applicable Interest Rate (as a percent)		4.36%
Outstanding Balance		$ 645,029,000	$ 667,502,000
Available for borrowing		581,833,000	567,167,000
Borrowings		116,200,000	213,000,000	$ 201,100,000
Payments on debt		$ 138,866,000	154,767,000	163,667,000
Bank Borrowings
Debt Obligations
Applicable Interest Rate (as a percent)		3.69%
Outstanding Balance		$ 112,000,000	96,500,000
Available for borrowing		488,000,000	503,500,000
Borrowings		116,200,000	113,000,000	123,600,000
Payments on debt		$ 100,700,000	123,600,000	137,000,000
Bank Borrowings | Subsequent Event
Debt Obligations
Outstanding Balance	$ 138,400,000
Available for borrowing	461,600,000
Borrowings	26,400,000
Senior Unsecured Notes
Debt Obligations
Applicable Interest Rate (as a percent)		4.50%
Outstanding Balance		$ 533,029,000	571,002,000
Available for borrowing		93,833,000	63,667,000
Borrowings			100,000,000	77,500,000
Payments on debt		$ 38,166,000	$ 31,167,000	$ 26,667,000
Senior Unsecured Notes | Subsequent Event
Debt Obligations
Outstanding Balance	528,862,000
Available for borrowing	98,000,000
Payments on debt	$ 4,167,000