Fair Value Measurements - Financial instruments not carried at fair value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 172,900	$ 173,000
Deposit liabilities	982,963	1,042,677
Notes payable and debt	3,240,461	3,220,769
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	172,879	172,982
Deposit liabilities	982,963	1,042,677
Notes payable and debt	3,240,461	3,220,769
Financial Liabilities	4,223,424	4,263,446
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	181,349	182,222
Deposit liabilities	1,190,885	1,266,682
Notes payable and debt	3,483,428	3,507,108
Financial Liabilities	$ 4,674,313	$ 4,773,790