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Convertible Securities - Additional Information (Details) - Junior Convertible Securities
 12 Months Ended
Dec. 31, 2017
USD ($)
trading_day
$ / shares
Debt Instrument  
Stated interest rate 5.15%
Principal amount at maturity | $ $ 50
Conversion ratio (in shares) 0.25
Conversion price (in usd per share) | $ / shares $ 200
Redemption closing price trigger (in usd per share) | $ / shares $ 260
Number of trading days closing price has exceeded threshold | trading_day 20
Number of consecutive trading days | trading_day 30
Deferred tax liability | $ $ 7,200,000