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Fair Value Measurements (Narrative) (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
loan
Dec. 31, 2016
USD ($)
loan
Dec. 31, 2014
USD ($)
loan
Cash and cash equivalents $ 398,900,000 $ 999,100,000  
Impaired loans 5,800,000    
Specific reserves and other write downs on impaired loans 1,800,000    
Troubled debt restructured loans balance 1,839,000 $ 1,866,000  
Troubled debt restructured loans, specific reserve $ 534,000    
Number of troubled debt restructured loans | loan 7 4  
Extended maturity $ 144,000 $ 144,000  
Minimum [Member]      
Estimated selling costs 7.00%    
Maximum [Member]      
Estimated selling costs 10.00%    
Walnut Street [Member]      
Loans, face value     $ 267,600,000
Price paid for a portion of the commercial loan portfolio     209,600,000
Notes payable     $ 193,600,000
Number of notes | loan     2
Fair Value, Measurements, Nonrecurring [Member]      
Impaired loans $ 4,011,000 $ 3,685,000  
Senior Notes [Member] | Walnut Street [Member]      
Senior notes     $ 178,200,000
Debt Instrument, Interest Rate, Stated Percentage     1.50%
Junior Subordinated Debt [Member] | Walnut Street [Member]      
Junior Subordinated Notes     $ 15,400,000
Debt Instrument, Interest Rate, Stated Percentage     10.00%