Fair Value Measurements - Financial instruments not carried at fair value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 173,200	$ 173,000
Deposit liabilities	940,557	1,042,677
Notes payable and debt	3,217,123	3,220,769
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	173,224	172,982
Deposit liabilities	940,557	1,042,677
Notes payable and debt	3,217,123	3,220,769
Financial Liabilities	4,157,680	4,263,446
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	181,551	182,222
Deposit liabilities	1,144,027	1,266,682
Notes payable and debt	3,393,394	3,507,108
Financial Liabilities	$ 4,537,421	$ 4,773,790