Holding Company Only Financial Statements (Condensed Unconsolidated Statements of Operations) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Income Statement [Abstract]                                    
Interest $ 22 $ 19 $ 18 $ 17 $ 14 $ 11 $ 11 $ 9 $ 8 $ 31 $ 36 $ 38 $ 39 $ 44 $ 31 $ 68 $ 39 $ 144
Income (loss) before income taxes 69 45 71 74 50 15 (38) 38 (118) (248) 14 61 23 129 124 240 (103) (149)
Benefit for income taxes (22) (12) (24) (28) (18) (4) 10 (12) 40 410 0 6 0 (43) (46) (82) 34 416
Net income (loss) 47     46   11 (28) 26 (78) 162 14 67 23 86 78 158 (69) 267
Preferred stock dividend/accretion           0 0 0 (1) (2) (1) (1) (1)     0 (1) (6)
Net income (loss) from continuing operations   $ 33 $ 47 46 $ 32 $ 11 $ (28) $ 26 $ (79) $ 160 $ 13 $ 66 $ 22          
Other comprehensive (loss) income (8)     (16)                   (21) 0 (6) 13 (3)
Comprehensive income (loss) $ 39     $ 30                   $ 65 $ 78 152 (56) 264
Parent Company                                    
Income Statement [Abstract]                                    
Interest                               7 7 7
General and administrative                               13 5 9
Total                               20 12 16
Loss before undistributed loss of subsidiaries                               (20) (12) (16)
Income (loss) equity in undistributed of subsidiaries                               172 (63) 247
Income (loss) before income taxes                               152 (75) 231
Benefit for income taxes                               6 6 36
Net income (loss)                               158 (69) 267
Preferred stock dividend/accretion                               0 (1) (6)
Net income (loss) from continuing operations                               158 (70) 261
Other comprehensive (loss) income                               (6) 13 (3)
Comprehensive income (loss)                               $ 152 $ (57) $ 258