Employee Stock Benefit Plans (Cash Settled Restricted Stock Unit Activity) (Details) - Cash Settled Restricted Stock Units (CRSUs) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested at beginning of period	17,067	116,712	115,687
Granted	0	16,299	1,025
Vested	(5,080)	(115,944)	0
Forfeited	(1,092)	0	0
Nonvested at end of period	10,895	17,067	116,712
Weighted-average grant-date fair value of stock appreciation rights granted using the Black-Scholes model	$ 0.00	$ 3.57	$ 36.96
Compensation expense	$ 49	$ 1,504	$ 317
Cash paid to settle cash settled restricted stock units		$ 2,300